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Index of Subjects On 20-Oct-06, at 12:57 PM, David & Alison Webster wrote: > > Steve Shaw wrote: >> Chris, Angus, others... >> Since the mimicry is superficial, extending to external bodily >> appearance and perhaps to some behaviour, it is unclear why mimicry >> is not distributed more or less evenly among other groups of flies, >> or why it is so common in syrphids and strats: it is hard to imagine >> that either group would have had some special 'preadaptation' not >> found in other families of fly, that would make mimicry a likely >> evolutionary strategy for the whole family -- at least I can't think >> of anything. > > Hi Steve & All, Oct 20, 2006 > I don't see any great mystery here. Mimicry to resemble potential > danger (bees & wasps) is a special case of widespread adaptations; > disguise to resemble something else [e.g. leaf, thorn, twig, sea weed, > seed, dirt, flower, background, danger, disagreeable taste & toxin to > name a few that come to mind offhand]. Agreed, no problem so far. > Evolution is a random walk; either in small circles, into greener > pastures or off of a cliff (metaphorical cliff in the case of flying > animals). Not sure of the entire metaphor here but I don't think so, for the random walk idea (which has a specific meaning as well). The direction that evolutionary species adaptations can take is actually very limited, depending very much on what was programmed into the original species by its particular genetic construction, interacting with its local 'environment' during development (epigenetic factors). So an insect's compound eye can't just suddenly evolve into a camera eye (and hasn't, although there have been successful moves in that direction by some day-flying Leps and even by the firefly beetle, of previous haiku-fame in the post of Brian's). A couple of families of fly, parasites, have gone on to lose their wings, but none has gone backwards to re-develop two wings again like those of nearly all the other insects, and like the paired-wing dipteran ancestors (possibly Mecoptera, scorpion flies). That's how strong developmental/genetic constraints can be, but even this is not irreversible in principle. As you probably know, mutations have turned up or have been induced in the fruitfly _Drosophila_ that can transform whole organs from one type to another, for instance from an antenna to a leg, and from a haltere back to a second set of wings on the thorax, so called homeotic mutations. So there is strong evidence of developmental control of transformation at a much higher level, of the sort that presumably occurred when an ancestral scorpionfly had its back wings converted to fly-type halteres way back in the Triassic, in the opposite direction, where they've stayed ever since. Copies of the genes involved in this control are found throughout metazoan animals and are now much studied (the genes are 'homologous' -- have a common ancestor -- rather than being completely identical, and the number of copies present in the genome varies). > In the case of relatively large insects, that spend appreciable time, > in full view, working flowers (Syrphids and Stratiomyids), is it not > reasonable that chance modifications, such that some strains resembled > dangerous insects, would confer some survival value ? Not all syrphids or strats are large, but otherwise it is a perfectly reasonable idea so far, and the 2nd and 3rd steps to resemble the model even more closely are also reasonable. > If all flies had taken this turn, then yellow with black would have > come to mean 'harmless food' and bees and wasps would have assumed > some other coloration. Good point -- one type of mimicry supposedly is only useful when it is a rare commodity; if everybody does it, the insect eating birds learn the rule very easily and it loses its usefulness (red-black is also a common warning coloration). This suggests why bee-wasp mimicry by flies should be uncommon, which perhaps explains why its is rare in most families, which in turn points up the question of why it is so common in syrphids: I think the original question remains valid. Next we ought to mention Batesian and Mullerian mimicry but that's getting deeper in... Also, Lep people know about this stuff in certain tropical butterflies: I think it is danaids (danaiids?) that use mimicry a lot. So why don't nymphalids, etc? > In addition, many late summer flowers are yellow, so yellow has a > certain background value. > Yours truly, Dave Webster, Kentville Yes, but wouldn't it be opposite? You'd want the tasty fly to stand out from the background, not blend in, to have it advertise its dangerousness. Anyway, I don't think many flower-visiting insects resemble their flowers, except for ambush artists like the crab spider (not an insect). Steve
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