Making Solomon's mines

As it seems to be the season for finding fly larvae (for example here and here), I thought I'd continue with this theme, but move from aphid predators to a species feeding on (or rather, in) plant material, in particular, a leaf miner feeding on Solomon's-seal (Polygonatum multiflorum).

Leaf mine in Polygonatum multiflorum - the arrows indicate small groups of feeding larvae

There are few leaf miners of P. multiflorum, and it was clear that these were most likely the larvae of the fly Parallelomma paridis (Diptera: Scathophagidae). However, a closer look was needed for confirmation, and having kept the leaf in a container overnight, the larvae began to leave the mine.

Larva of Parallelomma paridis. The head is to the bottom of the photo (note the black mouthparts) and the pair of posterior respiratory processes (PRPs) can be seen at the rear. Yellow-green gut contents are also visible.

Before looking more closely at the larva and subsequent pupa, it is worth noting that there has been some taxonomic confusion regarding this species. It has been synonymised with P. vittatum, but is now (by some dipterists at least e.g. Nelson, 1990 and Chandler, 1998) considered a separate species with P. vittatum on Orchidaceae (as noted by Smith, 1989) and P. paridis on Liliaceae as here. I take the view of Nelson and Chandler that these are separate species. The genus has also been known as Americina (which, along with the required Latinisation, means that some specimens of this species may be labelled Americina vittata).

The mines of this genus start near the eggs which are laid in small groups with the larvae then feeding communally.

Empty eggs of P. paridis - note the elongate shape and longitudinal ridges. The slit to the right appears to be the entrance to the mine

A small group of P. paridis larvae feeding together in the leaf mine. The arrows indicate the small, black, hooked mouthparts.

The mouthparts are hard 'sclerotised' hooked structures (mouth-hooks) which equate to mandibles and, in some groups of flies, are useful in identification. In this species the overall form is curved with a distinct hook at the tip. They are used to scrape leaf material into the mouth - the group of larvae work more or less in unison, gradually extending the mine as seen in the short video below the next photo.

Close-up of the mouth-hook of P. paridis. As the photo was taken of a live specimen still feeding in the mine, it is a little unclear (the rectangular leaf cell walls are visible), but the approximate outline on the right shows the hooked tip at the bottom of the image.

As well as muscle contractions, movement through the narrow mine is aided by the additional grip created by rings of tiny spikes on the surface of the larva, running around the front edge of each segment.

The head of the P. paridis larva - note the rings of tiny black spikes around the front of each segment. A pair of 'eye-lash' shaped tufts (probably with a sensory function) can also be seen, if a little out of focus, on the first segment.

Pitkin et al. (2012) state that the larvae of P. vittatum (noting the taxonomic confusion mentioned above) develop rapidly, emerging from the mine and pupating outside their food-plant after only 13 days and remaining as pupae for almost a year (around 348 days), with adults emerging in May and June. In fact 5 of the 6 larvae in my sample have already pupated and the other appears to be undergoing this process (see below) and so I expect to be storing the pupae for some time before being able to see the adults.

Pupation, with the associated hardening and darkening of the cuticle, appears to begin with the PRPs and then progress inwards from both ends of the larva. So, I would like to finish with a series of images showing the progress of pupation in this species and some of the structures of the pupa itself.

Larva of P. paridis with the PRPs having darkened as pupation is about to begin.

Larva of P. paridis showing partial pupation.

Pupa of P. paridis (approx 4mm long)

Head of P. paridis pupa showing the 'eye-lash' shaped tufts also seen in the larva above

Rear of P. paridis pupa showing the pair of PRPs and several smaller spines

Rear of P. paridis pupa showing the pair of PRPs and rings of small spikes around the articulations of segments.

Rear of P. paridis pupa showing the pair of PRPs and smaller lateral spines

References

Chandler, P.J. (1998). Checklists of insects of the British Isles (new series) part 1: Diptera. Handbooks for the Identification of British Insects 12(1): i-xx; 1-234.
Nelson, M. (1990). Observations on the biology and status of British dung flies of the genus Parallelomma Becker (Dipt., Scathophagidae). Entomologist's Monthly Magazine 126: 187-189. 
Pitkin, B., Ellis, W., Plant, C. & Edmunds, R. (2012). The Leaf and Stem Mines of British Flies and Other Insects: Parallelomma vittatum (Meigen, 1826) [accessed 05/06/2012].
[Diptera: Scathophagidae]
Smith, K.G.V. (1989). An introduction to the immature stages of British flies. Handbooks for the Identification of British Insects 10(14): 1-280.