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. In addition to the larva of _Sialis lutaria_, the first larval stage has also been found in the larva of _Ephemera_, _Tanypus_, _Corethra_, and _Chironomus_; the second in _Carabus hortensis_ Fabr., _Procerus_ (_Carabus_) _coriaceus_ Linn., _Calathus fuscipes_ Goeze, _Molops elatus_ Fabr., several species of _Pterostichus_, and a number of other beetles. It is probable that its normal hosts are _S. lutaria_ and _Pt. niger_, but it is clear that it often comes to rest in other insects. The view that the Gordiidae have no special hosts, but may either pass the whole of their life-history within one and the same animal, or, on the other hand, may inhabit animals belonging to very different groups, is held by Villot, who has paid great attention to the subject. He finds the first larval form encysted in the walls of the alimentary canal in fishes, such {173}as _Leuciscus phoxinus_, the minnow, _Cobitis barbatula_, the loach, and _Petromyzon planeri_, the lamprey; in the larvae of Diptera, _Ephemera_, and beetles, in _Planorbis_ (a water snail), in _Enchytraeus_ (an Oligochaet); the second larval form in all kinds of insects, spiders, Crustacea, fish, frogs, birds (_Otis_), and in man, and these various habitats lead him to the conclusion that "Les Gordiens n'ont pas d'hôtes spéciaux." On the other hand, as von Linstow points out, it is contrary to our knowledge of parasites that a single species should develop equally well in the body of warm and cold-blooded Vertebrates and of Insects, and the explanation of the presence of the larvae in these various forms may either be that they belong to different species of _Gordius_ or, more probably, that they are accidentally present, having passed into their hosts with drinking water.