ANATOMY OF IXSECTS, 283 



It lias already been remarked that the thorax bears the three pairs of jointed limbs which are 

 characteristic of Insects. One pair of these organs is appended to each segment of this division of the 

 body, and thus the original division into three segments may be indicated even, when the amalgamation 

 of the segments themselves appears to be most complete. These limbs are inserted into sockets 

 on each side of the sternum by means of their first joints, which may be spherical, or nearly so, 

 and thus enable the limb to turn in any direction, or more or less elongated or ovate in form, when 

 the movement of the limb from the socket will be more like that of a hinge. These first joints, called 

 coxce (Fig. 3, a), are followed \>y a piece usually of small size, the trockanter 

 (6), which may be a ring-like joint uniting the coxa to the following joint 

 of the leg, or a more or less triangular plate extending along part of the 

 under surface of the latter. The third piece, generally the largest and most 

 powerful joint of the leg, is the thigh or femur (c), at the apex of which Fl o- 3. WALKING LEG OF 

 the shank or tibia (d) is articulated by a hinge joint. Both these parts, but 



especially the tibia, are frequently armed with spines and bristles ; the tibiae in particular are 

 very commonly furnished at their extremities with movably articulated spurs, which project 

 considerably, and materially assist the insect in walking. The actual foot, or tarsus (e), is. 

 attached to the extremity of the tibia, and is composed of a variable number of joints, but 

 never more than five. The joints of the tarsi often vaiy much in size and form, even in 

 the same insect or in the same tarsus ; they are generally clothed beneath with short stiff hairs, or 

 modifications of hair-like structures, and the last joint bears at its apex a pair of movable claws, 

 between or beneath which in the majority we find small membranous appendages, which are called 

 pseudomjchia, or arolia. 



These jointed limbs are modified in a great variety of ways, and their peculiarities of form and 

 structura are of much importance in the classification of insects. Their chief use being for walking 

 or running, they are, in the majority of these animals, subject only to minor modifications of the- 

 whole organ, or some of its parts ; but where special functions have to be performed by them, 

 the changes are much greater. In this way the legs, or some of them, may be strongly compressed and 

 widened, and provided with strong fringes of bristles to fit them for natatorial purposes, as in the 

 Water-beetles ; or shortened and thickened, furnished with great cylindrical coxae, broad, toothed 

 tibia} and short concealed tarsi adapting them to the purpose of digging, as in the Mole-crickets ; 

 or elongated, and provided with very powerful thighs for jumping, like the hind legs of Grasshoppers 

 and Locusts ; or furnished with very long coxae, thighs grooved and spined beneath, and tibiae and 

 tarsi arranged to fit into the groove of the thighs, rendering them formidable prehensile organs, 

 of which the fore legs of the so-called Praying Insects (Mantida?) are examples. Of the infinity of 

 minor modifications, elongations, or abbreviations of parts, partial dilatations and contractions, 

 development of bristles and spines, &.G., there is no occasion to speak here ; examples enough of them 

 will have to be described in characterising the insects in which they occur. 



Besides the legs the thorax usually bears one or two pairs of wings in fact, in the majority 

 of adult insects the whole of the organs of locomotion are confined to this region of the body. The front 

 pair of wings when there are two pairs spring from the mesothorax, and the second pair from the meta- 

 thorax, and always from the junction between the dorsal plate (notum) and the pleurae. They 

 are generally thin membranous organs, and notwithstanding their delicacy they consist always 

 of two membranes continuous at their edges, and firmly attached to each by their inner surfaces- 

 This peculiarity of construction is explained by the mode of formation of the wings. These are 

 originally sac-like dilatations of the integument, which gradually become extended and more 

 delicate in texture, until the inner surfaces come in contact and adhere one to another; but this does 

 not finally occur until after the insect has arrived at maturity, and the two membranes, of which the 

 wing is composed, can be at first easily separated. In most cases the wings are traversed by a 

 greater or less number of veins, which branch in various ways, and generally form a sort of network. 

 These veins consist primarily of air-tubes, similar in construction to those which ramify through 

 all parts of the body, and serve, as will be described hereafter, for the purpose of respiration, 

 and passing out of the thorax into the wings, spread out there between the two membranes. They 

 are usually thickened by an increased deposition of horny chitine along their course, and as 



