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This is where I post various musings about wildlife and ecology, observations of interesting species (often invertebrates)
and bits of research that grab my attention. As well as blogging, I undertake professional ecological & wildlife surveys
covering invertebrates, plants, birds, reptiles, amphibians and some mammals, plus habitat assessment and management
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Thursday, 24 May 2012

Great hovering maggots

When collecting brambles to feed our stick-insects, small invertebrates on them often end up indoors as stowaways. As the bramble leaves are destined to be rapidly eaten, I tend to put the invertebrates back outside - recently I've unexpectedly found moths, aphids, a parasitic bee, small spiders, barklice and others. Some of these get out of the stick-insects' containers and wander about the place. So, I wasn't too surprised this morning when I saw something wiggling its way across the lid. The pointed head end with a dark point and pulsing legless 'maggoty' movement made it clear this was a fly larva - in particular a hoverfly (Syrphidae).

Hoverfly larva, the head end is at the bottom of the photo
Being a bug-nerd, before putting it back outdoors, I decided to take a closer look to see if I could identify it. Using the standard work by Rotheray (1993), it is clearly not Microdon (which is domed in shape and has a band of bristles aruond the edge of the body), but does have a colour pattern. It's not green (though the fat deposits are a yellow-green colour), nor does it have dorsal projections, although there is some dorsal striping. The next step is to look at the 'posterior respiratory projections' (PRPs) at the rear end.

Rear end of the hoverfly larva showing PRPs with pale bases and about as wide as long.
The larva is somewhat flat and so 'subrectangular' rather than square-ish in cross-section, and has dorsal stripes merged smoothly at the head end - these features indicate that it is in the genus Platycheirus.

Head end of Platycheirus larva showing the point where dorsal stripes meet smoothly
Platycheirus larvae are associated broadly with low or ground-layer vegetation and feed on aphids - definitely popular with knowledgeable gardeners! However, the ecological and biological details of Platycheirus larvae, including differences in feeding strategies between species, are poorly understood. This does not have clear chevrons of fat so is probably in the fulviventris group, although not P. fulviventris itself as this is associated with wetland monocots rather than hedgerow brambles. Consulting Stubbs & Falk (2002), fulviventris is in the clypeatus group and it may well be the common and widespread P. clypeatus although the only way to be certain would be to raise it to adulthood and the habitat is more suited to another common species P. albimanus although this falls outside the fulviventris and clypeatus groups. Fortunately for the larva, it is back outside on aphid-laden bramble in our garden.

Dorsal view of the Platycheirus larva showing the dark gut between patches of fat

References

Rotheray, G.E. (1993). Colour guide to hoverfly larvae (Diptera, Syrphidae). Dipterists Digest 9: 1-156.
Stubbs, A.E. & Falk, S.J. (2002). British Hoverflies. BENHS, Reading.

1 comment:

  1. Interesting blog. I was drawn to this particular post as my main interest is hoverflies. Alas, I have some bad news… your larva is actually a Syrphus sp.

    The larva doesn’t have the well-defined longitudinal dorsal stripes as you see on Parasyrphus and Platycheirus fluv for example, so you would select the next couplet in Rotheray’s key that describes the larva as “larva translucent with white, yellow or pale brown markings”. This would take you to Key III.5. Your larva lacks the pair of rounded posterior projections found in Trichopsomyia and the prp does have dorsal spurs (I can just about see them in your photo but they aren’t clear and I have a fair amount of experience of Syrphus to help me here). Next, we find that it lacks the basal rim found on Meliscaeva auricollis (which is actually a pale looking larvae that might cause confusion with pale forms of Syrphus but since yours is a yellow form we can also discount that species based on colour). Then you reach the couplet where you have to decide whether the prp is broader than long (or vice versa). You had already noted it is the former so we can now conclude that your larva is a Syrphus sp!

    Rotheray’s key is difficult to use at the first attempt and the only reason I can navigate the keys now is because I’ve reared a fair number of larvae of several genera over the years! I hope this helps…

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