Page:Encyclopædia Britannica, Ninth Edition, v. 13.djvu/156

144 INSECTS inouth-covering is the "lingua" or tongue. The same general arrangement is present throughout all insects, and also in the larval and pupal stages ; but the differing conditions of the food cause extreme modification, not only between differing groups or orders of the perfect insects, but also in the rnetamorphic stages of the one and the same species. In some insects there are additional small structures, such as the " paraglossse." The " thorax" is the next main division. It is composed of three distinct portions, the prothorax, mesothorax, and metathorax, all subject to excessive modifications; but the last is, on an average, the smallest ; at any rate it seldom exceeds the intermediate, and is usually very much smaller. According to surface, each portion receives two different names; thus the upper side consists of the pronotum, mesouotum, and mctanotum, the lower of the prosternum, mesosternum, and metasternum. It will be noticed also that each subdivision is again subdivided by more or less distinct grooves, especially above and on the sides, indicat ing its compound nature, and each of these has its special term, so that some authors go so far as to say that each thoracic division is formed of nine separate pieces (a text book on entomology, which this article cannot be, should be consulted as to these). In those insects in which the wing-power is great, the attachments of the muscles are strongly indicated externally. The appendages of the thorax are the legs and wings. The legs are articulated members, of which one pair is attached to the sides of each subdivision. All true insects have but six actual legs, but in the larvae of some orders there are simple fleshy prolegs on the abdominal segments, considered as representing the homologues of those abdominal legs so conspicuous in the Myriopoda. Of the true legs the anterior (or prothoracic) pair are directed forward, the two other pairs backward. Each leg consists of a basal joint or coxa (frequently not movable) inserted in sockets termed the acetabula ; this is followed by a small joint termed the trochanter placed between the coxa and femur or thigh, which is ordinarily the largest joint, and is enormously developed in saltatorial insects. To this succeeds the tibia, followed again by the tarsus, which is ordinarily compound, but may consist of any num ber of joints from one to five. The tarsus is terminated by a pair (seldom one only) of claws, between which are more or less membranous arolia or plantulse (much marked in the feet of Diptera, which climb polished surfaces, &c., by means of them), and also a pulvillus or cushion. Wings are appendages of the mesothorax and metathorax (never of the prothorax), and, viewed simply as organs of locomotion, may be considered as expansions of the integu ment, though some morphologists object to this simple definition, and one at least (F. Plateau) regards them as tracheal extensions. Although in all orders there are cases in which they are never developed, the exceptions being so few as abundantly to prove the rule, yet the posterior (or " hind " or " under ") pair may be absent, and the anterior (or "fore" or "upper") ample. So strongly are they attributes of a perfect insect, that in some cases in which neither pair is developed the creatures strongly incline to retain their larval form. Normally the first external indications may be said to appear in the pupal stage (but we will show that in insects with imperfect metamorphoses the line of demarcation between larva and pupa is not marked), and they only attain their full development some little time after the exclusion of the perfect insect. A wing consists of an upper and lower membrane (readily separable in a recently excluded insect, or afterwards by maceration), strengthened by more or less numerous strong ribs, more or less connected transversely, termed nervures or veins (neither term being very appropriate), which are chitinous tubes (containing special trachea}), through which the blood circulates. The varying condition of the wings will be alluded to in the systematic portion of this article, as also to some extent the scheme of neuration, one of the most important factors in systematic entomology, but rendered unsatisfactory in consequence of the utterly different nomenclature employed by writers on special orders, though doubtless the general scheme is capable of being homologized. The last of the three great divisions of the body is the abdomen, which consists of a number of segments (nor mally nine), having an upper (dorsum) and lower (venter) chitinous surface, which two surfaces (in the most char acteristic condition) are connected by a membranous lateral line, with lateral stigmata or spiracles. But almost every conceivable modification is presented both in its attachment to the thorax, its general outline, and the number of segments present. Of the appendages of the abdomen it is necessary to say but little. In a perfect insect there are no abdom inal legs, and rarely any indications of breathing plates (so usual in some groups of aquatic larvae). The appendages are therefore almost entirely connected with the sexual apparatus, which vary enormously, and occasionally there are terminal articulated thread-like tails, strongly simulat ing antennae both in form and structure. Nervous System. This may be said to consist of a more or less double cord lying along the ventral portion of the body, connected at intervals by thickened masses termed ganglia. But the large mass in the head is termed the brain, in contradistinction to the others. The brain usually consists of a bilobed mass giving off nerve masses to the eyes, and threads to the other cephalic appendages or organs ; recent researches prove that, at any rate in some cases, the brain has convolutions analogous to those of the higher animals. Immediately below the brain is a large ganglion, usually termed the infra-cesophagal, con nected with the mouth organs and digestive functions. Then follow, in the thorax and abdomen, a series of ganglia, each of which gives off numerous lateral threads. But the number of these ganglia varies very greatly, not only in insects of different orders, or in species of the same order, but also in the larvae, pupse, and perfect insects of the same species ; and it is impossible to enter here into the most rudimentary analysis of these variations. It has been said that normally there should be a ganglion for each segment (or for each movable segment) of the body, and to some extent this would appear to hold good, for, in those insects in which some of the segments coalesce, a similar arrange ment is seen in the system of ganglia, but this would not appear to be a universal law, and in some the abdominal ganglia are virtually obsolete. Similar variations exist in the extent to which the double central column becomes united or remains divided. In addition to this column, a simple sympathetic nerve is also distinguished, without ganglia, but giving off threads to the respiratory and other systems. This lies above the main gauglionic chain. In minute structure the nervous cord of the Tnsecta is analo gous to that of higher animals. The simplicity of the nervous system has caused it to be believed that insects do not suffer pain in the sense of that experienced by higher animals, and their behaviour when subjected to treatment that should cause intense pain, in the ordinary sense of the word, appears to warrant such an opinion ; but the existence of such a condition cannot bo held to justify wanton cruelty. Those who desire minute informa- tioi, on the nervous system should especially consult New port s article "Insecta" in Todd s Cydopxdia of Anatomy and Physiology, and a series of articles by E. Brandt, now appearing in the publication of the Russian Entomological Society.