Fam. 21. Nepidae.—Abdomen furnished behind with a long slender siphon; front legs more or less elongate for capturing prey, placed quite at the front edge of the prothorax.—This family consists of two interesting but very dissimilar genera, Nepa and Ranatra. Both are widely distributed over the earth, and are rather numerous in species.[[495]] We have one species of each genus in Britain. Nepa cinerea, the common "water-scorpion," is one of the commonest of Insects in Southern Britain, living concealed in shallow waters when nearly or quite stagnant. Ranatra linearis (Fig. 276) is much less common, and appears to be getting rarer; it is not recorded from farther north than Cambridge.
Fig. 276.—Ranatra linearis, with the two portions, a, of the respiratory siphon separated. Cambridge.
The nature of the respiratory arrangements in these Insects is of considerable interest; the long tube at the extremity of the body consists of two parts (as shown in Fig. 276) brought together in the middle, one from each side. Lacaze-Duthiers states that the processes are elongated pleurae, but in the young it is far from clear that this is the case. However that may be, they seem to convey air to the true breathing organs, situate inside the cleft on the apical part of the abdomen itself; but details as to the way in which transfer of air is effected along this very protracted passage are not forthcoming. The development in Nepa has been studied to a certain extent. The apical stigmata are the only pair of the abdominal stigmata that exist in the imago of Nepa, the other six pairs being obliterated; the third, fourth, and fifth, according to Schiödte, in a very peculiar manner: hence, as Martin says,[[496]] the respiratory system is metapneustic. In an earlier stage of the life, however, these six pairs of stigmata exist in functional activity placed in a groove on the under surface of the body; so that the condition is that termed peripneustic, and remains so till the final moult, when the long siphon appears. In the early life there is a short prolongation from the end of the body in connection with the pair of grooves alluded to, but it is a single unpaired organ, and does little therefore to explain the appearance of the siphon, which must, at present, be considered as being suddenly developed at the last moult.
Fig. 277—Egg of Nepa cinerea. (After Korschelt.)
The eggs of Nepidae are remarkable objects; that of the common water-scorpion bears seven filaments at one end (Fig. 277); while that of Ranatra is more elongate, and bears only two, very elongate, threads. These eggs are deposited in the stems of water-plants, being introduced therein, so that the body of the egg is concealed while the threads project: those of Ranatra are placed in stems floating on the water, and in consequence of the threads the stems look as if they were infested by some fungus. The structure and formation of the eggs have been investigated with considerable detail by Korschelt.[[497]] He looks on the filaments as pneumatic, and considers that they supply a coating of air to the body of the egg; they consist of a spongy mass encircled by two layers of egg-shell, both of these latter being peculiar in structure; the spongy mass is continuous with a layer of the same kind of substance placed on the interior of the shell of the body of the egg. It will be recollected that we have described (p. 562) an egg, apparently of the same nature, deposited by Capsids in the stems of land plants, so that it is very doubtful whether the threads are really connected with the aquatic development of the embryo in Nepidae. But the most interesting feature connected with these eggs is, according to Korschelt, the mode of development of the filaments, which is sui generis; the shell of the egg is developed in the ordinary manner as an exudation or excretion from epithelial cells; but the shell of the filament is formed as an intracellular product; a mode of chitin-formation that appears to be peculiar to this structure. Korschelt remarks that "it is in the highest degree worthy of attention how by any process of development through a large number of successive generations so complex a condition could be established as the result of adaptation to external conditions; and this becomes even more interesting when we remember that highly peculiar special processes and departures from the usual modes of tissue-formation are necessary to permit the development of this apparatus."[[498]]
Fam. 22. Naucoridae.—No ocelli, and no terminal process to the body; front legs inserted on or near the front of the prosternum. Anterior femora usually broad and flat.—The members of this family are truly aquatic, and swim readily in the water. The family is small, including about nine genera and thirty species, but, like many water-Insects, the genera are widely distributed. We have two in Britain—one of them, Naucoris, common; the other, Aphelocheirus, rare.
Fam. 23. Belostomidae.—No ocelli, and no long terminal tube to the body; front legs inserted near the front of the prosternum. Posterior tibiae not spiny; flattened and provided with swimming hairs.—Although these Insects have been classified with Nepidae they have but little relation therewith; on the other hand, the distinctions from Naucoridae are far less important. The family includes some of the largest Insects. The South American Belostoma grande attains a length of four or four and a half inches. Notwithstanding their considerable size Belostomidae exist in very large numbers in some localities, and frequently destroy young fish by aid of the powerful though short rostrum.
