Making my daily journey to and from work on foot means that I sometimes come across interesting insects and plants along my route. The other morning, for instance, I came across a freshly dead Great Diving Beetle by the side of the path. This diving beetle is one of our most impressive species, measuring in at a staggering 36 mm in length and quite chunky with it. I guessed that a passing car had hit this one sometime during the night. Great Diving Beetles are strong fliers and having spent the winter at the bottom of a suitable waterbody, this one had clearly emerged to search for a mate. Dispersal of these wandering adults mostly happens during the night and they can sometimes turn up at lighted windows (or moth traps) attracted by the light. Once the male finds his mate the pair will copulate – a process that can last for several hours – some time after which eggs will be laid.
Because this particular individual was dead there was no problem in handling it. A living diving beetle, however, can inflict a painful nip and the needle-like spurs on the legs can draw blood if the beetle is not handled with care. The beetle also has one other means of defence up its sleeve and this is a chemical one. In response to being handled a Great Diving Beetle will not only turn out the contents of its rectum onto your hand but it will also produce a milky fluid from glands on its prothorax. This fluid smells unpleasant and, remarkably, is made up of a cocktail of compounds which include certain steroids more commonly associated with vertebrate animals. For many would-be predators, notably fish, the compounds are distasteful and the fish will quickly release a beetle it has grabbed. It has also been discovered that some of the compounds act as neurotoxins and are sufficiently strong as to kill a number of would-be amphibian predators. Perhaps most surprising of all is the quantity of some of these vertebrate steroids in the beetles. It would, for example, require the adrenal glands from several herds of cattle if you wanted to extract a similar dose of the steroid cortexon to that found in a single beetle.
This particular beetle is the most widespread of the seven big water beetle species (those over 20 mm in length) and it can be found in many different types of waterbody. However, it has a preference for still or slow-moving waterbodies, so had probably emerged from the river, just a few yards away, or one of the lakes that are located a little further upstream.