Small, spiny and very very hungry

It must be the time of year, but after writing about a Platycheirus hoverfly larva, I found another one - a different genus, but the overall form and fused posterior respiratory processes (PRPs) make it clearly a hoverfly (Syrphidae); larvae of some other fly families such as the Pipunculidae have PRPs that appear fused, but a close look reveals they are not.

Dorsal view of the hoverfly larva - about 10mm long and the head end is bottom left.

This is quite different to the smooth, 'maggoty' Platycheirus larva with its yellow-green lines of fat. Here, small bumps and spines can be seen on the top and sides plus lots of little black speckles (see below). The PRPs are a pale brown colour and the dark line down the middle is the long thin 'heart' which could be clearly seen pumping while I took this shot. Referring as ever to Rotheray (1993), the features soon brough me to two possible genera, either Scaeva or Eupeodes (previously known at Metasyrphus). Let's look a little more closely...

PRPs and posterior clasper of Eupeodes luniger

Close-up of PRPs and posterior clasper of E. luniger. The green polygon outlines the clasper and the red lines follow the directions of the lower pair of spiracles, showing that they diverge strongly.

The photo legends give the game away - it's Eupeodes luniger - but let's consider why. Firstly, the top photo of the PRPs (themselves within a roughtly triangular surround) shows that the black patches are made of tiny dark spines or 'spicules' - something that separates Eupeodes from Scaeva. Most Eupeodes are found on conifers, but two are commonly associated with ground layer vegetation - E. corollae and E. luniger. The larvae of these (as with Platycheirus they are aphid predators) are fairly similar, but can be separated by looking carefully at the spiracles - the slit-like openings of the PRPs. In corollae the lower pair are almost parallel, but in luniger, as here in the lower photo, they diverge strongly. The clasper is also typical of Eupeodes and, as shown clearly in Rotheray (2003), is used to grip the edges of leaves or thin stems (here it is gripping a thin wooden spatula being used to position it under the microscope). This is a surprisingly unusual adaptation in hoverfly larvae many of which do sometimes fall off leaves as they can not easily grip them and have to rely on the surface tension created by moisture on leaves; some exude saliva just ahead of themselves and use this to grip (Rotheray & Gilbert 2011). The locomotory structures of Eupeodes are also known to be complex (besides the presence of a clasper) and the photo below hopefully gives some indication of the folding and creasing that forms the prolegs and associated structures on the underside of the larva. Though migratory, both E. luniger and corollae are common in a range of habitats in Britain, and their similarity to Scaeva remains when adult. This specimen, as before, has now been released into our garden to feed on aphids. More soon!

The complex prolegs and locomotory growths on the underside of Eupeodes

References

Rotheray, G.E. (1993). Colour guide to hoverfly larvae (Diptera, Syrphidae). Dipterists Digest 9: 1-156.
Rotheray, G.E. (2003). Aphid Predators. Richmond, Slough.
Rotheray, G.E. & Gilbert, F. (2011). The Natural History of Hoverflies. Forrest Text, Tresaith.