Faking stinkers: Do bug nymphs mimic harvestmen?

A fairly short (well, short-ish) post for once, but one that asks a question that's been niggling at me for a while - do bug nymphs mimic harvestmen? At first glance this is not a question with a clear foundation, and certainly falls within the realm of 'speculative biology' (a place I don't visit that often) - bug nymphs don't look anything like harvestmen which have long spindly legs attached to a central round-oval body. However, by 'bugs' I'm only thinking of the Coreidae ('squashbugs') and Pentatomoidea ('shieldbugs'), and only one aspect of harvestmen (Opiliones) - the ocularium (eye-bearing structure) on top of the body. Let's look a little more closely...

A 'typical' harvestman of the genus Mitopus.

A North American harvestman, courtesy of 'bugman' at What's That Bug?

Both of these show the oval body on top of which is the small ocularium - a raised structure with two laterally oriented eyes which incidentally, despite appearing large like spider eyes, are ocelli and do little more than register light-intensity - touch is a far more important sense in harvestmen (Hillyard 2005). The top picture also shows the central dark band or 'saddle'. OK, so we know what harvestmen look like - but what's this got to do with bug nymphs? Well, let's see...

Late-instar nymph of the coreid Coriomerus denticulatus

Late-instar nymph of the coreid Arenocoris falleni

Final-instar nymph of the Juniper Shieldbug Cyphostethus tristriatus

[All three bug photos are copyright Tristan Bantock at the excellent British Bugs guide to British Hemiptera which has an extensive gallery including other nymphs showing these markings, though not all species do. For example, nymphs of the familiar Dock Bug Coreus marginatus do not show these 'ocularium' markings.]

Looking at these three nymphs, I can't help but wonder if the structures on the top of the abdomen are mimicing the ocalarium of harvestmen. To me, they certainly appear similar, especially in the coreids which share similar colouration and segmentation (superficially at least) with the harvestmen, though of course my poor human brain is wired to look for patterns, so simple appearance may not tell me much. I can do little more than speculate at present, and am unaware of anyone else who has looked at this, but the phrase 'Batesian mimicry' leaps to mind. This is the type of mimicry where a harmless species mimics one that is dangerous or unpalatable (for example, there are many species which mimic the colours of wasps). How might this suggested example be Batesian?

Well, bugs are likely to be potential prey of many insectivores, and during their nymphal stages cannot fly - so, mimicry might be useful, but why harvestmen? Although predatory, mainly on small soft-bodies invertebrates, harvestmen have no venom and are unlikely to be able to inflict much damage on potential predators such as birds, spiders, beetles, centipedes, fish, frogs and shrews. However, they do possess odoriferous glands (also called 'repugnatorial' or 'stink' glands) which are found on the sides of the carapace, approximately level with the ocularium. These glands produce a spray or droplet than the harvestman can spread on itself or an attacker. Though the chemicals (various alcohols, ketones and naphthoquinones) are not always easily detected by humans' sense of smell, there have been various observations made of potential predators avoiding (e.g. ants and spiders) or expelling (e.g. frogs) harvestmen because of their distastefulness (Hillyard 2005). Might this form the basis of mimicry by bug nymphs?

At present this is mere speculation derived from observation - something that might be formulated as a hypothesis - and some initial questions are raised such as whether the bug nymphs themselves are distasteful, in which case this would be an example of Müllerian, rather than Batesian, mimicry (as both groups have anti-predation characteristics). It is something that I will have to dig into further and as ever I welcome thoughts and suggestions.

Reference

Hillyard, P.D. (2005). Harvestmen (3rd ed.). Field Studies Council, Preston Montford. A small but excellent book, and the current standard work on British harvestmen.

Further reading

If you are interested in squash- and shieldbugs in Britain, this is excellent:

Evans, M. & Edmondson, R. (2005). A Photographic Guide to the Shieldbugs and Squashbugs of the British Isles. WGUK.