DARWIN'S WARM LITTLE POND. A candidate place for the origin of life on earth*.

The warm little ponds I consider here are rain pools of a particular type - that is those on rock surfaces as apposed to those on soft substrata which are quite different (Jackson & McLachlan, 1991). Furthermore I consider only those found in Malawi, which is in tropical Africa and consider only the principal inhabitants - midge larvae of three remarkable species at prodigious densities and strictly only one of the three in any particular pool (McLachlan & Ladle, 2001). Aspects of the ecology and adaptive biology of inhabitants are discussed in published work and in my web site (google.atholmclachlan.blogspot.com). I repeat only sufficient to set the scene for my present endeavour which is to emphasise some aspects of the pools that may not have been considered previously or that have not been fully appreciated previously. To start I will attempt to meet comments I have had from editors and others that have lead me to believe that not everyone grasps what they are seeing in photographs of these rain pools. Perhaps they are expecting something bigger and more conspicuous. So, pictures of selected pools are provided in pairs, those on the top are unadorned, while those below have been crudely coloured using a computer app. My classification of pool types depends primarily upon the shape of the depression on the rock surface that accommodates the pool. Pool depth is the principal determinant of pool duration after rain which in turn largely determines which midge species inhabits the pool as larvae (Cantrell & McLachlan, 1982). Thus, in terms of duration from shortest to longest we have - Polypedilum vanderplanki (Fig. 1), Dasyhelia thompsoni (Fg.2), and the two species of Chironomus. The Chironomus species are themselves dived by proximity to permanent water (rivers). C imicola inhabits pools remote from rivers (Fig. 3), while C. pulcher occurs only near rivers where there is shade (Fig. 4). Some C. pulcher pools will be created when filled with river water rather that rain water as the river recedes during the dry season. But, it seems improbable that there will be any ovipositing females to colonise them during the dry season. Or am I wrong? P. vanderplanki alone is able to survive as a completely desiccated larva when its home pool dries though D. thompsoni larvae can survive in specially constructed cocoons against the dry rock bottom. Chironomus larvae cannot survive desiccation and must emerge as adults and leave the home pool before it dries (Cantrell & McLachlan, 1982)

Fig. 1, Pools on a rock surface occupied by the larvae of Polypedilum vanderplanki.

Fig. 2. A pool inhabited by larvae of Dasyhelia thompsoni pools.

Fig.3. Chironomus imicola is found in sunny pools, typically on hill tops, such as the one in Fig. 2.

Fig. 4, Typical pools, p1 and p2, inhabited by larvae of C.pulcher, near a receding river, r. v, vegetation and e, rock surface. Sandra McLachlan on the right again provides scale. To conclude I consider some potentially important interactions with other animals. These include birds, mammals, frogs, ants and rotifers. Birds, notably crows, are frequent visitors to drink and may be important in transporting inhabitants between pools. Birds were cited by Charles Darwin in the role of transporters (Darwin, 1859), and it would be interesting, if a way could be found of doing it, to investigate their role in the present context. Mammals, notably civets and genets, play an unexpected part for they use pools as lavatories (McLachlan, 1981a). Thus it may be a mammal’s choice of pools, rather than pool duration that is the primary determinant of the pools suitability for the larvae of the midge D. thompsoni. Larvae of this species may be able to tolerate the presumably toxic condition following genet activity. Some specialised frog species use the pools as breeding sites (Patterson & McLachlan, 1989). There is an interesting twist to the feeding behaviour of the tadpoles of these frogs because they feed on the water surface where algae and wind-born pollen are to be found. Faecal pellets resulting from their feeding activities fall to the pool bottom where they are accessible to the Dipteran larvae. Thus tadpoles are effectively feeding the dipteran larvae by providing food otherwise inaccessible to larvae (McLachlan, 1981b). Furthermore, when a pool dries, tadpoles are often trapped and die. But there is more because some midge larvae, notably those of D. thompsoni, are able to use dead tadpoles as food when the pool refills after rain (McLachlan, 1981b). The larvae of P. vanderplanki are not safe even when entering a desiccated when their home pool dries because in this condition they provide ideal biltong for scavenging ants who carry them off to their nests (McLachlan & Cantrell, 1980). The abundant rotifers in P. vanderplanki pools do not turn up in P. vanderplanki gut samples so may not interact directly with dipteran larvae, as food at least. I have largely neglected the question of the role of adults emerging from a pool in choosing new pools suitable for oviposition. It is the adult that is responsible for pool choice and we do not know how this is done. Perhaps water chemistry, conditioned by the previous generation of larvae plays a role, but would this work when flushing by thunderstorms presumably removes water ‘conditioned’ by larvae. Nevertheless, an hypothesis that conditioning by larvae triggers oviposition by the appropriate female is, in principal, readily tested. I would create artificial pools, all of the same dimensions, one half with conditioned water and, as control, the same number with unconditioned water. The prediction here is that females would only oviposit in pools conditioned by larvae of the same species as the female. There is much to be learned about the role of adult midges in pool choice.

Fig. 5. These pools in Botswana are about the same depth (duration), as those inhabited by Chironomus in Malawi but are inhabited by crustacea instead. Botswana lies in the subtropics while Malawi is in the humid tropics so it is biogeography to which we must look for an explanation of this difference. Pools inhabited by crustacea have attracted their fair share of attention (Vanschoenwinkel et al., 2011). References *Cited by Dawkins, R. (2004), p465. The Ancestor's Tale. A Pilgrimage to the Dawn of Life. Weidenfeld & Nichlson. Jackson, M. J. & McLachlan, A. J. (1991). Rain pools on peat moor land as island habitats for midge larvae. Hydrobiologia, 209, 59-65. Cantrell, M. A., & McLachlan, A. J. (1982). Habitat duration and dipteran larvae in tropical rain pools. Oikos, 38, 343-348. Darwin, C. (1859). On the Origin of Species by Means of Natural Selection or the Preservation of Favoured Races in the Struggle for Life. (6 ed.). London: John Murray. McLachlan, A. J. (1981a). Food Sources and Foraging Tactics in Tropical Rain Pools. Zoological Journal of the Linnean Society, 71, 275-277. McLachlan, A. J. (1981b). Interaction Between Insect Larvae and Tadpoles in Tropical Rain Pools. . Ecological Entomology, 6, 173-182. McLachlan, A. J., & Cantrell, M. A. (1980). Survival Strategies in Tropical Rain Pools. Oecologia, 47, 344 - 351. McLachlan, A. J., & Ladle, R. (2001). Life in the puddle: behavioural and life-cycle adaptations in the Diptera of tropical rain pools. Biological Reviews 76, 377-388. Patterson, J. W., & McLachlan, A. J. (1989). Larval Habitat Duration and Size to Metamrphosis in Frogs. Hydrobiologia, 171, 121-126. Vanschoenwinkel, B., Mergey, J., Pinceal, T., Waterkeyn, A., Vandewaerde, H., Seaman, M., et al. (2011). Long Distance Dispersal of Zooplankton Endemic to Isolated Mountain Tops - an Example of a Ecological Process Operating on an Evolutionary Scale. PLos one, 6, 1-10.

SUNLIGHT

Exposure to sunlight appears to be an underappreciated aspect of habitat. Among non-living features of habitat, it is water and temperature that usually come to mind. This note attempts to show that ignoring sunlight is probably a mistake. Indeed sunlight, it appears, is often the main aspect of habitat that determines the distribution of animals and plants. I recently came across a discussion of the role if sunlight in Helena Cronin’s book, The Ant and the Peacock (1991), pp91-92. There she discusses the findings of such distinguished zoologists as Arthur Cain and P. M. Sheppard, on the degree of exposure to sunlight in determining the distribution of the iconic snail Cepea nemoralis. She goes on to show that Alfred Russell Wallace himself took a deep interest in the role of sunlight. Their work together provides context, which I had not at first fully appreciated, for my own effort to identify habitat selection in the two rain-pool dwelling species of chironomid midges, Chironomus imicola or Chironomus pulcher, Life in the Puddle (1988). Why are some rain pools consistently occupied by either C. pulcher or C. imicola, presumably for thousands or even millions of years? Evidence is accumulating that it is the degree of exposure to sunlight responsible here too. Some typical rain pools recently filled by rain, on rock surfaces in Africa are shown above.

Alternative Phenotypes

 

The Evolution of Migration Routes in Birds and Insects

 

Consider an archipelago. Island inhabitants within an archipelago gain in fitness by moving between islands or between islands and mainland. This is done to exploit regularly changing conditions. Such behavioural adaptations lead to the evolution of migration routes. Migration, often over spectacular distances, is well known for many bird and mammal species. Typically, migration routes follow genetically fixed compass bearings as shown for birds in Fig. 1d, from (Davies, Krebs, & West, 2012).

 

 

 

Though we usually picture islands as land surrounded by water, reflection leads to the important generalisation that essentially all habitats are isolated patches surrounded by inhospitable terrain. In other words they are islands. Different species have different ecological requirements, so will perceive different places as habitable islands.

 

      In this essay I want to consider rain pools. There are many kinds of rain pool but all are transient habitat islands drying up frequently. When full they are exploited by a variety of aquatic animals, in prodigious numbers endemic to their kind of pool. Examples include pools in the foot prints of animals such as elephants, those in the axils of leaves, in pitcher plants, in tree hollows and on the surfaces of rocks (McLachlan & Ladle, 2001). It is the last of these, specifically those on sheets of rock in tropical Africa (Fig. 2), that is the focus of my interest here.  

 

FIG. 2. An example of pools (P1 and P2), near a river (r), flowing over a sheet of rock in Malawi in tropical Africa. In general such pools may be true rain pools while others are filled by river water as the river recedes during the dry season. From (McLachlan, 1988).  

 

 

There is quite a lot known about the ecology of pools on rock surfaces in Africa (hereafter called rock pools), and the adaptations of the indigenous animals inhabiting them (McLachlan & Ladle, 2001). I here contend that adaptive migration behaviour as seen in birds can be expected to evolve among the inhabitants of rock pools but not among the inhabitants of other ephemeral habitats. This is because rock pools are always to be found in the same place, which is not true of other rain pools. I return to this point below.

 

Principal among the inhabitants of rock pools are aquatic insect larvae that cannot survive the drying of their home pool. Individuals that manage to reach adulthood before the home pool dries must migrate to find a pool with water in which to oviposit. I here consider two such species, both midges in the Genus Chironomus. i.e. C. pulcher and C. imicola. Note that for rain pool species there is strictly one species in any particular pool and the association persists, when the pool is full, from year to year and presumably for thousands, even millions of years. It is worth noting too, that the genus Chironomus is an African genus of invaders, quick to exploit new opportunities but poor at competition with other species (Armitage, Cranston, & Pinder, 1995). In this sense these two rock pools breeding insects are preadapted (M. J.  West-Eberhard, 1992) p15.

 

 

 

     Now for some details:  C pulcher is confined to pools near rivers. Such pools are effectively permanent because even in the dry season new pools are formed as the river recedes, exposing new pools full of river water. The pools in Fig 2 are of this kind. During the rains, pools remote from rivers also fill and provide opportunities for invaders. It is these remote pools that are exploited by C. imicola. C. pulcher does not move from the archipelago of near–river pools, which are refuges for both species in the dry season. Figs. 3 and 4 below show the sampling sites where the work discussed here were done between 1970 and 2004.

 

 

 

 

FIG. 3. The study area, seen from the NW, is indicated by a broken line. It includes a heterogeneous landscape of rivers (r) and mountains (m) rising to a height of 250m above thinly wooded plains (dark patches are cloud shadows). X indicates the highest mountain within the study area. The photo was taken from 800m up Zomba massive. Mulanji massive (M), rising to a height of 2300m above the plain is barely visible in the far background. C, Chancellor College, University of Malawi.

 

FIG. 4. The distribution of pools inhabited by Chironomus pulcher (black dots), and those inhabited by Chironomus imicola (red dots), (a) in a dry season and (b) in the next wet season. Each point represents a single rock pool within the sampling are shown in Fig. 3. Pools dry at the time of sampling are not shown. Mountains are represented by contours at 50m intervals. The red arrow shows a predicted wet season migration route for C. imicola from river refuges. The principal refuge, measured as number of full pools at the time of sampling is indicated by a red circle.

 

 

I have made the point that rock pools are different from other rain pools and even different from other ephemeral insect habitats such as fallen fruit, carrion and the dung of mammals and that the evolution of migration routes can therefore be predicted for insects indigenous to rock pools, here C. pulcher and C. imicola. How might the evolution of migration routes have originated for these two species? The proximate requirement is that migration direction is genetically determined and that migration direction varies with the population. Individuals with migration direction resulting in the successful location of appropriate remote pools pass on their migration direction genes to the next generation, thus leading to the evolution of migration routes. Note that I am not here proposing adaptive phenotypic flexibility (M. J. West-Eberhard, 2003), but well understood genetically fixed phenotypic behaviour (Davies et al., 2012).

 

The hypothesis of migration routes for C. imicola can, in principal, be tested by tagging with a radio isotope such as P³². The whole population of larvae in a pool is tagged and allowed to reach adulthood. Egg masses from all pools within the sampling area are later collected to determine the distribution of P32 labelled egg massed from migrating females.

 

 

 

Now we arrive at last at my central point. Because of their spatial predictability over geological time scales, rock pools, possibly along with deep sea fumaroles, may be habitats unique on this earth. Their spatial relationships are presumably not even interrupted by tectonic place movements. We should not forget either, that rock pools are candidate places for the origin of life on earth (Mclachlan 2017). For these reasons it would be interesting to determine the phylogeny of these two strange rock pools breeding species. I predict that they will prove to be sister species, but if an hypothesis of sister species is rejected another interesting conclusion is indicated. I mean the independent evolution of two species adapting to an ephemeral habitat with low levels of interspecific competition and high levels of spatial predictability (McLachlan & Ladle 2001).

 

 

references

Armitage, P., Cranston, P. S., & Pinder, L. C. V. (1995). The Chironomidae. The biology and ecology of non -biting midges. . London: Chapman & Hall.

Davies, N. B., Krebs, J. R., & West, S. A. (2012). An Introduction to Behavioural Ecology. (4 ed.). Oxford: Wiley-Blackwell.

McLachlan, A. J. (1988). Refugia and habitat partitioning among midges (Diptera: Chironomidae) in rain-pools. Ecological Entomology, 13, 185-193.

McLachlan, A. J., & Ladle, R. (2001). Life in the puddle: behavioural and life-cycle adaptations in the Diptera of tropical rain pools. Biological Reviews 76, 377-388.

McLachlan, A. J. (2017). The living puddle. http://atholmclachlan.blogspot.com.

West-Eberhard, M. J. (1992). Adaptation: Current Usages. In E. Fox Keller & E. A. Lloyd (Eds.), Kewords in Evolutionary Biology. London: Harvard University Press.

West-Eberhard, M. J. (2003). Developmental Plasticity and Evolution. Oxford: Oxford University  Press.

 

 

Some Influential Colleagues and Ideas

Influential colleagues and Ideas

Some Influential Colleagues

In September 1970 my wife Sandra and I immigrated to the UK from South Arica, for me to take up a lectureship in the Zoology Department of Newcastle University. This essay is about how this came about and the role of the Head of Zoology, Professor Robert Clark in bringing it about.

Bob, as we called him, was ever a remote figure as was usual in the days when the title ‘Professor’ meant something, so I was never privy to his plans for me or for the Zoology Department. Nevertheless I own him a great debt of gratitude for bringing me to the UK with its professional opportunities. Conjecture leads me to the conclusion that, probably encouraged by Professor Leonard Beadle, Bob brought me to the UK to develop ecology as a major subject in the Zoology Department. At the same time Dr Stewart Evans was appointed to carry out the same role for animal behaviour. Such a plan would have made sense as both ecology and behaviour were subjects popular with students in a department previously dominated by physiology and palaeontology. I was to join Norman Philipson a senior ecologist already in post. I was to learn later how lucky I was to have Norman as a friend and colleague. Norma and I were provided with a large modern laboratory with capacity for at least a dozen research students and a full time staff demonstrator post. Here was a grand and generous innovation owed entirely to Bob. 

                                                          Professor Arthur Cain FRS

                                                                     Dr Alec Panchen

                                                              Professor Leonard Beadle

                                                          Professor Margaret Kalk

Some transformative Lectures

Striking in my memory is an Arthur Cain lecture on systematics given as a guest lecture at Wits University Zoology Department in the late 50s when I was an undergraduate. In those days, at least at Wits, undergraduate work was heavily focused on systematics and comparative anatomy so Cain’s lecture was timely for me in bringing together these two themes. Strangely no one had attempted to do this for us before. Dr Cain, as he was then, was a fine orator, making his subject both exciting and memorable.

 I cannot neglect a lecture by a Dr Brink from the Bernard Price Palaeontology Institute at Wits entitle The Thermal Barrier. We students were familiar with the concept of the water barriers to the colonisation of land but the idea of a thermal barrier was new to us. So stimulated were we that some of us, including my future wife Sandra Bosazza and my brother Ian both made palaeontology a focus of their interests thereafter.

A guest lecture by the eminent John Maynard Smith, another fine orator, turned everything on its head with a title something like A Chicken is the Eggs Way of Making Another Egg. This is typical of him. A great innovator, he is responsible for the theory of evolutionary stable strategies (EES) and Game Theory which reinforced my budding interest in natural selection.

A Book

I add here one book of considerable influence entitle Ontogeny and Phylogeny (Gould, 1977). S. J. Gould’ moves away from the adult centric zoology in fashion for many years, to a focus on the life cycle of organisms. This is just what I needed to move my study of the ecology of the common chironomid midge from the feeding larva stages to the reproducing adult (McLachlan & Ladle, 2001)( McLachlan, 2013).

References

Beadle, L. C. Obituary. Arch. Hydrobiol. 108, 583-587.

Cain, A. (1999). Obituary. Nature, 401, 872.

Gould, S. J. (1977). Ontogeny and Phylogeny. London: The Belknap Press of Harvard University Press.

Kalk, M., McLachlan, A. J., & Howard-Williams, C. (Eds.). (1979). Lake Chilwa: Studies of change in a tropical ecosystem (Vol. 35). London: Dr. W. Junk.

McLachlan, A. J., & Ladle, R. (2001). Life in the puddle: behavioural and life-cycle adaptations in the Diptera of tropical rain pools. Biological Reviews 76, 377-388.

McLachlan, A. J., & Ladle, R. (2011). Barriers to Adaptive Reasoning in Community Ecology. Biological Reviews, 86, 543-548.

Williams, G. C. (1966). Adaptation and Natural Selection. Princeton: Princeton University Press.

Wynne-Edwards. (1962). Animal Dispersion in Relation to Social Behaviour. Edinburgh: Oliver & Boyd.

Winter Gnats

Winter Gnats

Fig 1 with the help of Andrew Mortley

These flies (Trichocera), look rather like chironomid midges (Fig. 1), and, also like chironomids (McLachlan & Neems, 1995), form mating swarms to acquire a mate. Chironomids are well known because of dominance of the larval stages in populations of many, perhaps most, lakes and streams (Armitage, Cranston, & Pinder, 1995). But does anyone know anything about winter gnats? They are not mentioned in Gullan and Cranston (Gullan & Cranston, 1994) but do appear in Imms (Imms, 1957), with a brief description, on p. 610. To anyone used to chironomids the proportions of wing to body are all wrong. Evidently the morphology is like that of the Tipulidae, to which they are related. What interests me about these flies is their ability to swarm on very cold December days. Indeed this is the only time mating swarms are seen. There are clear adaptive advantages to flying under these conditions because of the absence of insect predators such as bibionids and empids. Does their morphology, in contrast to the chironomids, provide an explanation? I want to suggest that the relatively large wings act as solar panels, picking up radiant heat even at very low air temperatures. Indeed the possibility of insect wings functioning as solar panels has been proposed by Reynolds (2020,) p159, as an intermediate step in the evolution  of insect flight. 

Turning to the adaptive aspects following from the possibility of wings as solar panels, some conjecture is possible. For example, the Trichoceridae share their distinctive morphology with another major family, the Tipulidae or crane flies. Both families have exceptionally long legs and exceptionally large wings (Imms, 1957), pp. 608-610). As far as I know, of these two families, it is only the trihocerids that have a swarm based mating system (McLachlan & Neems, 1995). It is within this type of mating system that the large wings may have a fitness advantage permitting conspicuous mating swam to function in the virtual absence of invertebrate predators. We need to know something of the phylogeny of these two families but it is conceivable that they share a common ancestor. In which case large wings came first and can be regarded as a pre-adaptation (Dawkins, 1996), p.95, to mating in predator free conditions.

Fig. 1. Composite drawing to illustrate the relative wing to body size of a male chironomid midge (a), and a winter gnat (b). Legs not shown.

Fig. 2. A sketch of the tether with a fly in position – a, stand; b, wire; c, drop of ‘typex’; d, midge or gnat. The attachment ‘c’ must leave the wings free of interference.   Details are from (McLachlan, 1983), p550.

An hypothesis that wings act as solar panels is, as far as I can ascertain, a novel one. Furthermore it is testable. Here is the design of a simple experiment.

Treatment: Fly winter gnats in tethered flight in a controlled temperature cabinet at various low temperatures.  A lamp provides solar radiation.

Control:  Fly chironomids under the same conditions. Choose species of approximately the same body size as the gnats in the treatment.

Treatment control: A treatment control is required with the light as a source of heat

 radiation removed.

Unknowns: Will gnats fly on a tether? I have not attempted this. I know chironomids perform well under these conditions (McLachlan, 1983).

Field work: Record air temperatures and solar radiation in mating swarms in the wild for both gnats and midges. Results should inform the treatment temperatures.

If this works it will be a neat little experiment but it may not be as straight forward as it seems. There could be other, perhaps better experiments to test the same hypothesis. 

references

 Armitage, P., Cranston, P. S., & Pinder, L. C. V. (1995). The Chironomidae. The biology and ecology of non-biting midges. . London: Chapman & Hall.

Gullan, P. J., & Cranston, P. S. (1994). The Insects: An Outline of Entomology. London: Chapman & Hall.

Imms, A. D. (1957). A General Textbook of Entomology. (9th ed.). London: Methuen.

McLachlan, A. J. (1983). Life-history tactics of rain-pool dwellers. Journal of Animal Ecology, 52, 545-561.

McLachlan, A. J., & Neems, R. M. (1995). Swarm based mating systems. In S. R. Leather & J. Hardie (Eds.), Insect Reproduction. New York: CRC Press.

Dawkins, R. (1996). Climbing Mount Improbable. London: W. W. Norton.

Imms, A. D. (1957). A General Textbook of Entomology. (9th ed.). London: Methuen.

McLachlan, A. J., & Neems, R. M. (1995). Swarm based mating systems. In S. R. Leather & J. Hardie (Eds.), Insect Reproduction. New York: CRC Press.

Renolds, S. (2020). Success! When, Why and How insects got their wings. Bull. Ent. Soc. Antenna, 44, 155-160.

Adaptation to habitat unpredictability and some consequences

The subject of this essay is rain pools – specifically those on rock surfaces in tropical Africa. Rain pools are at the extreme of short duration among fresh waters, some lasting just hours after rain. Yet they are inhabited by immense populations of midge larvae – strictly a single species to each pool. Over the past thirty five years, on and off, I have been interested in adaptations enabling these extraordinary animals to survive and flourish in what would appear an unpromising place. In the end this work has hinged on a comparison of the adaptations of two principal inhabitants, larvae of Chironomus species on the one hand and larvae of Polypedilum vanderplanki, on the other. What has not previously received attention are the consequences following from adaptation to habitat desiccation. I refer to exposure to parasites and predators.

For the first time I consider costs as well as benefits to adaptive strategies to habitat unpredictability. To develop my arguments I adopt the comparative method practised by John Crook and by Peter Jarman, among others (Krebs & Davies, 1993), pp. 25-29. In other words, I compare strategies and in so doing attempt to bring out the relationships between ecology and accompanying adaptation. As pointed out by John Krebs and Nick Davies 1993, p. 25, this is rather like looking at the results of an experiment done over evolutionary time with species acting as controls for each other.

To begin, some background.  The rain pool faunas include three species of midge, Chironomus imicola, Chironomus pulcher and Polypedilum vanderplanki. I focus on the two Chironomus species taken together in contrast to P. vanderplanki. For my present purposes we need to know only that Chironomus inhabit pools with an average duration close to the average duration of the aquatic larval and pupal stages. P vanderplanki, on the other hand, inhabits pools with an average duration many times shorter than the average duration of the aquatic stages. Chironomus spp have behavioural adaptations enabling them to use cues to imminent pool desiccation and are triggered by these cues to metamorphose and leave the pool as adults. These cues allow, in effect, predictions of what is likely to happen in the future. In this sense adaptation can hinge on future events, a much misunderstood matter (Dawkins, 1986), p71. P. vanderplanki by contrast, survive in the dry home pool as desiccated larvae. As would be expected the choice between stay and leave is universal wherever habitat duration is unpredictable. Maynard Smith, pp262-263, cites the case of fish in transient habitats, for example, lung fish (stay), vs. osteolepid fish (leave) (Maynard Smith, 1975).

I turn now to the costs of the parasites and predators associated with the stay or leave strategies. Both parasites and predators infect aquatic stages of the midges but costs occur in the adult stage. The predicted costs and benefits of the stay or leave strategies are shown in Table 1.

Table 1. Costs and Benefits in terms of changes in fitness due to interactions with predators and parasites. The predators considered are ants and parasites, worms and mites.  

           Taxon                                         cost                                        benefit

Chironomus	Parasites	?
P. vanderplanki	Predators	No parasites

Entries in Table 1 take the form of testable hypotheses. My reasoning is as follows: costs to Chironomus accrue to adults harbouring either mites or worms (McLachlan, 2006). For P. vanderplanki, costs are due to predation by ants mining for larvae in suspended animation (Fig. 1). Benefits accrue because P. vanderplanki individuals escape parasites which are unable to infect desiccated larvae. (Fig. 2) shown a mite (Unionicola ypsilophora) and a worm probably Gastromermis rosea. To a first approximation, I regard both as parasites.

Fig. 1. Top. Spoil tips from ant mining activity in the bed of a dry P. vanderplanki rain pool.

Below left. An ant with a larva in its mandibles. Below right. Two ants fighting over a larva in cryptobiosis. From (McLachlan & Cantrell, 1980).

Fig. 2.  Parasitic mites and worms. a,  a female midge baring several mites. b,  a females midge with a large worm (g), emerging.

All hypotheses are readily tested, for example by capturing adults emerging from pools in the wild to count parasites. Negative fitness effects to ant predation can be determined by counting larvae removed by ants against the number recovering after artificially flooding the home pool.

In conclusion I wish to broaden the scope of this essay and in so doing promote a better understanding of the adaptive landscapes to which the insects are exposed.

I start with Chironomus and the question over the benefits shown for Chironomus (Table 1). Pools do not invariable dry before the full growth of Chironomus larvae is complete (McLachlan & Ladle, 2001). The benefits to Chironomus larvae when the home pool is full include access to super-abundant food with virtually no inter-specific competition. But whenever larvae succeed in metamorphosing there is a considerable cost, i. e. the burden of parasites in the adult stage of the life cycle (McLachlan, 1999). So, for Chironomus, the frequency of early desiccation must be brought into the equation (McLachlan, 2006).

Turning to P. vanderplanki, these larvae enjoy the same benefits as Chironomus when pools contain  water. There is a cost in the larval stage to predication, the extent of which remains to be measured but it can be predicted that there is a very considerable fitness benefit to the absence of parasites. There is more to anhydrobiosis. Referring to my (2013) Blog, it has been suggested that anhydrobiosis is the reason for some desiccation resistant rotifers dispensing with both sex and parasites. As far as can be determined they never have sex! Perhaps, as suggested by Hamilton and Zuk (1982), they don't need sex in the absence of parasites. 

References

Dawkins, R. (1906). The Extended Selfish Gene. Oxford University Press. 

Krebs, J. R., & Davies, N. B. (1993). An Introduction to Behavioural Ecology. Oxford: Blackwell Scientific Publications.

Maynard Smith, J. (1975). The Theory of Evolution. (third ed.). New York: C. Nicholls 

McLachlan, A. J. (1999). Parasites promote mating success: the case of a midge and a mite. Animal Behaviour, 57, 1199-1205.

McLachlan, A. J. (2006). You are looking mitey fine: parasites as direct indicators of fitness in the mating system of a host species. . Ethology Ecology & Evolution 18, 233-239.

McLachlan, A. J., & Cantrell, M. A. (1980). Survival Strategies in Tropical Rain Pools. Oecologia, 47, 344 - 351.

McLachlan, A. J., & Ladle, R. (2001). Life in the puddle: behavioural and life-cycle adaptations in the Diptera of tropical rain pools. Biological Reviews 76, 377-388.

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