Snow one reason for mimicry
IN the native forest reserves at Peel Forest during summer, an odd little fly with a black thorax, smooth nonhairy orange abdomen and amber blacktipped wings flies conspicuously among grass in forest margins.
This fly, Huttonobesseria
verecunda (family: Tachinidae), a parasitoid of true bugs, differs markedly from most flies of its family, which have very bristly abdomens, dark bodies and clear wings.
H. verecunda, is a nonstinging Batesian mimic of stinging female solitary wasps, including spider wasps (Pompilidae) and ichneumonid wasps, which also have smooth abdomens and share its coloration.
There are many other mimics here, including beetles in three families and several families of true flies. Although they are edible, birds avoid these wasplike mimics.One robber fly (Asilidae) walks in the same jerky manner as a spider wasp, has golden hairs on its face and thorax, and wriggles its front legs in front of its face to mimic the spider wasp’s antennae, because unlike a spider wasp, a robber fly’s antennae are very short.
Nonstinging male spider wasps sometimes fly down to this robber fly, mistaking it for a female spider wasp, whereupon the fly pounces on the male wasp and eats it. Birds avoid the fly because it resembles a female spider wasp. It is thus an aggressive mimic to the male wasp and a Batesian mimicy to birds.
Females of several families of solitary wasps comprise rings of similarly coloured stinging Mullerian mimics (models).
The spider wasp
Sphictostethus fugax nests above ground, often in holes high in trees made by mediumsized woodboring beetles. It is abundant at Peel Forest, where unusually heavy falls of snow occur every 1520 years, the weight of snow breaking many branches. Female weevils of the genus Psepholax make a short 2.53mmwide burrow in the damaged bark, then reverse in and lay a series of eggs at the end.
On hatching, the larvae burrow inwards and then follow the grain of the wood. When fully grown, the larvae burrow towards the surface and make oval 8mmwide chambers in which to pupate, first cutting a hole to the exterior through which the adult beetle exits.
The spider wasp S. fugax nests in these pupal chambers, and is unusually common at Peel Forest because Psepholax holes are so plentiful.
Here also, many soldier fly larvae (Benhamyia apicalis [Stratiomyidae]) live under dead bark on damaged trees, the adult flies mimicking S. fugax, which nests close by. In this way, the periodic heavy falls of snow have a noticeable effect on the composition and appearance of Peel Forest insects.