The changes of form that accompany the growth of the individual are much less abrupt and conspicuous than they are in most other Insects. The metamorphosis is therefore called Paurometabolous. It has been supposed by some naturalists that Orthoptera go through a larger portion of their development in the egg than other Insects do. This does not clearly appear to be the case, though it seems that there are distinctions of a general character in the embryology; the period of development in the egg is prolonged, and the yolk is said by Wheeler[[126]] to be more than usually abundant in comparison with the size of the young embryo. The embryonic development may in tropical countries be accomplished in three weeks (see Mantidae), but in countries where winter supervenes, the period may in some species be extended over seven or eight months.
The external features of the post-embryonic development—a term that is more convenient in connexion with Orthoptera than metamorphosis—are as follows: the wings are never present when the Insect is first hatched, but appear subsequently, and increase in size at the moults; the form and proportions of the segments of the body—especially of the thorax—undergo much change; an alteration of colour occurs at some of the moults, and the integument becomes harder in the adult condition. Neither the development of the internal organs, nor the physiological processes by which the changes of external form are effected, appear to have been studied to any great extent.
Many of the Orthoptera do not possess wings fit for flight, and some species even in the adult state have no trace whatever of such organs. Flight, indeed, appears to be of minor importance in the Order; in many cases where the wings exist they are purely musical organs, and are not of any use for flight. The apterous and the flightless conditions are not confined to one division of the Order, but are found in all the families and in many of their subdivisions. As the front pair of wings in Orthoptera do not really carry out the function of flight, and as they differ in several particulars from the hinder pair, or true wings, it is usual to call them tegmina. The musical powers of the Orthoptera are confined to the saltatorial group of families.
Fig. 101.—Poecilimon affinis ♂. Bulgaria. Alar organs serving only as musical organs. The ear on front tibia and aural orifice of prothorax are well shown.
The Cursoria are dumb or nearly so; it is a remarkable fact that also in this latter division the alar organs, though frequently present, have but little value for flight, and are in some cases devoted to what we may call purposes of ornament or concealment. This is specially the case in the Phasmidae and Mantidae, where the effectiveness of colour and pattern of these parts becomes truly astonishing. The tegmina frequently exhibit an extraordinary resemblance to vegetable structures, and this appearance is not superficial, for it may be seen that the nervures of the wings in their disposition and appearance resemble almost exactly the ribs of leaves. One of the most remarkable of the features of Orthoptera is that a great difference frequently exists between the colours of the tegmina and of the wings, i.e. the front and hind wings; the latter are concealed in the condition of repose, but when activity is entered on and they are displayed, the individual becomes in appearance a totally different creature. In some cases, contrary to what usually occurs in Insects, it is the female that is most remarkable; the male in Mantidae and Phasmidae being frequently a creature of quite inferior appearance and power in comparison with his consort. The musical powers of the saltatorial Orthoptera are, however, specially characteristic of the male sex. There is evidence that these powers are of great importance to the creatures, though in what way is far from clear. Some parts of the structures of the body are in many of these musical species clearly dominated by the musical organs, and are apparently specially directed to securing their efficiency. We find in some Locustidae that the tegmina are nothing but sound-producing instruments, while the pronotum is prolonged to form a hood that protects them without encumbering their action. In the males of the Pneumorides, where the phonetic organ is situated on the abdomen, this part of the body is inflated and tense, no doubt with the result of increasing the volume and quality of the sound. In the genus Methone (Fig. 185) we find a grasshopper whose great hind legs have no saltatorial function, and but little power of locomotion, but act as parts of a sound-producing instrument, and as agents for protecting some parts of the body in repose. Further particulars of these cases must be looked for in our accounts of the different groups.
The eggs of many Orthoptera are deposited in capsules or cases; these capsules may contain only one egg, or a great many.
The Order includes many species of Insects, though in Britain it is poorly represented: we have only about forty species, and this small number includes some that are naturalised. Only a few of the forty extend their range to Scotland. A revision of the species found in Britain has recently been made by Mr. Eland Shaw.[[127]] In continental Europe, especially in the south, the species become more numerous; about 500 are known as inhabitants of geographical Europe. In countries where the face of nature has been less transformed by the operations of man, and especially in the tropical parts of the world, Orthoptera are much more abundant.
The lowest number at which the species now existing on the surface of the earth can be estimated is 10,000. This, however, is probably far under the mark, for the smaller and more obscure species of Orthoptera have never been thoroughly collected in any tropical continental region, while new forms of even the largest size are still frequently discovered in the tropics.
We shall treat the Order as composed of eight families:—