It is rather difficult to say what brings these essentially terrestrial beetles to the water, but von Linstow suggests that, as they live partly on snails, and at this time of year there are not many land-snails about, they may be in search of water-snails such as Limnaea. They may also be sometimes blown into the water by wind storms, but, whatever the cause is, their presence in water is essential for the continuance of the life of their parasites.
Once free in the water the Gordius is soon sexually mature; the fertilisation takes place in April, and then the female may be seen twisting and writhing round the stems of water-plants and laying the long bead-like strands of eggs (Fig. 82). The first deposition observed by von Linstow took place on 14th April, the last on 2nd August, and the period of egg-laying for each female extended over four weeks. At first the eggs are snow-white, but within twenty-four hours they turn brown in colour.
The development of the first larva within the egg takes about a month. When it emerges from the egg-shell it is minute, .065 mm. long, ringed anteriorly, and provided with a protrusible and retractile boring apparatus consisting of three chitinous rods; round the base of this piercing proboscis is a double crown of papillae, each bearing a spine (Fig. 90).
Fig. 89.—The tail ends of a female Gordius (a) and a male (b) in copula. × 1.5. (From G. Meissner.[[212]])
This first larval form breaks through the egg-shell and sinks to the bottom of the water, where it moves about sluggishly and awaits the arrival of the right host in which to take up its abode. This host is the larva of the Alder-fly, Sialis lutaria Lin. (vide vol. v. p. 444), and into this it bores and comes to rest in the muscles or the fat body. It does not form distinct capsules. It remains in this larva during the following winter, and in the spring passes over into the imago Sialis. The complete insect frequents the small plants growing along the water's edge, and falls an easy prey to the predaceous beetle Pt. niger. The larva is eaten, and undergoing a change becomes the second larval form mentioned above. It remains in the body of the beetle during the second winter, and finally returns to the water as the adult some eighteen or twenty months after it has been hatched from the egg.
Fig. 90.—Embryo or first larval form of Gordius tolosanus taken from the egg. Highly magnified. a and b, The bristle-bearing papillae on the head; c, the boring apparatus. (From von Linstow.)
From the above account of the life-history of Gordius it will be seen that the chances of an egg reaching maturity are comparatively small, and to compensate for this a very large number of eggs are laid. In addition to the risk of the larvae not finding the right host at the right time, and of the first host not being eaten by the second, and the second not being drowned, there is the danger that the ditches and ponds in which the adults live may dry up, and, in fact, great numbers of worms perish by this taking place.
The sex of the adults may be told from their colour, the males being of a blackish brown, the females of a light clay brown; the former average 120 mm. in length, the latter 170 mm. The males are also more numerous, the proportion being seven to three. Camerano[[213]] has drawn attention to the fact that there is a certain polymorphism in size, form, and colour which is especially common amongst the males; dwarf forms with mature reproductive organs exist, and he is of opinion that these differences depend both on the size of the second host and on the duration of the parasitic life.