Resident weevil...

A couple of days ago my wife brought in a weevil that she had found perched on washing in our garden (most domestic!), and wondered if I knew what it was. It looked familiar and looked like something quite common, but got me thinking about the identification and recording of common invertebrates. After all, many of them look very similar and can't be accurately identified from a straightforward inspection, even with a hand lens. This means that the less common species may well be under-recorded if the assumption is that the specimen is the more common of various initial possibilities. So, I thought I'd go through the process of identifying a small invertebrate, in this case the captured weevil.

The first thing to mention is literature - often books, sometimes online resources such as interactive keys - because without these, identification of many species with any certainty is difficult or more likely impossible. Personally I tend to buy what I can find/afford and borrow the rest (e.g. from biological records centres, museums, inter-library loans and so on). There is, for me anyway, great joy to be had in building up a good references library. Anyhow, onto the specimen - looking yellower than in reality due to lighting...

Dorsal view (approx 5mm long, excluding legs & antennae)

Ventral view

Now, the first stop (unless you already know which family/subfamily it's in) is Morris (1991) and the key to main groups of weevils. The first step's easy - it has elbowed antennae with a long first segment. Then onto the rostrum; it's short and this could cause confusion sending the identification towards Platypodidae and Scolytidae (which it isn't), but the tibiae are smooth on the outer edge, the thorax doesn't have borders and is also clearly narrower than the front of the elytra - so, it's in the Curculionidae ('true weevils'). Now for a closer look to get to subfamily...

Underside of the head

Side view of the head showing curved scrobes (dark antennal sockets in front of the eyes)

Scales on the elytra - pale and flat

Looking at the scrobes (middle pic), they can't be seen clearly from above, are narrow, and don't point in a direction passing through the middle of the eyes - so, not Otiorhynchinae. There's no clear fringe of hairs along the sides of the front edge of the thorax - so, not Tanymecinae. The elytra together are about twice as long as wide, and the upper surface has pale scales forming stripes on them - these features mean it is in the subfamily Sitoninae, now known as the Tribe Sitonini within the subfamily Entiminae (Duff 2008). It also mean it's time to leave Morris (1991) and move onto Morris (1997) which has keys to tribe, genus and species levels (and can also be used to key from Curculionidae to Sitonini, taking a slightly different route looking at tarsal claws).

Though Duff (2008) includes several genera within the Sitonini, Morris (1997) keys them to species but considers them all to be within the genus Sitona - mentioned just to add a little taxonomic confusion to the mix. Anyhow, keying onwards...

The scutellum has no upright tufts of scales and has a rostrum about as long as wide. So, not S. griseus. The elytra have flat scale and no upright bristles; the dorsal surface has a striped (rather than tesselated) pattern and is scaled rather than pubescent - the overall appearance is brownish rather than black. The head is also more-or-less flat between the eyes (rather than being deeply dented) and the upper edges of the eyes are not obviously higher than the top of the head. OK so far, but it now gets a little trickier.

I can't see the median furrow on top of the head very clearly, but it does broaden out and reach further back than the midpoint of the eyes, ending without a clear pit. So, not the scarce S. puncticollis. Also, the elytra are more than 1.5 times as long as wide, and more-or-less straight-sided, and the scales are oval but not 3-4 times as long as broad (more like 2 times), so it isn't the common S. lepidus. The eyes are clearly convex rather than almost flat which leaves just two species - S. cylindricollis and S. lineatus. The separation is straightforward enough here - in S. lineatus the pronotum is widest behind the middle, and slightly narrower at the base than at the front (as here) and also has complete elytral stripes rather than just short segments as would be seen in S. cylindricollis. So, this is an overwintering adult of S. lineatus just emerged in the spring - widespread and abundant in lowlands, fully winged, and known to feed on pea and bean leaves in gardens and allotments as well as on vetches. Eggs are laid by these overwintered adults with larvae feeding on root nodules and the next generation adult by about July in the south, August in the north. So, I suppose it could be seen as a pest, but a few leaf-holes won't matter and so it has been released back into the garden. I fully expect to find more...

References

Duff, A.G. (ed.) (2008). Checklist of Beetles of the British Isles. A.G. Duff, Wells.
Morris, M.G. (1991). Weevils. Richmond, Slough. [Naturalists' Handbook no. 16; an excellent introduction]
Morris, M.G. (1997). Broad-nosed weevils. Coleoptera: Curculionidae (Entiminae). Handbooks for the Identification of British Insects 5(17a): 1-106. [Royal Entomological Society; a bit more advanced]