How they are constructed 



parts. The names given in inverted commas, 'hip', 'thigh' and so on, 

 are the nearest equivalents in human anatomy. The tip of the tarsus 

 usually has a pair of claws, and often one, two or three adhesive pads. 

 The parts of the leg are labelled in Fig. 14. 



A segmented leg is primarily a device for walking or running, and for 

 this purpose is generally slender, and flexibly jointed. A great many 

 insects, however, use some or all of their legs for other purposes : for 

 jumping, or swimming; or not for locomotion at all, but for seizing and 

 holding their prey, for holding the other partner during mating, or for 

 producing sounds. A few even use the glands of the legs to produce 

 silk, or to extrude fluids that are poisonous, irritant or alarming to their 

 enemies. Some of these special uses of the legs are discussed in the 

 following chapters. 



The wings of insects are outgrowths from the wall of the thorax, 

 and have two surfaces pressed closely together, like a paper-bag that 

 has been flattened. The membrane that is thus formed is supported 

 by a series of ribs, or veins. One of these supports the leading-edge, 

 and the tip, and often the trailing-edge as well. Other veins branch 

 out from the base of the wing, and are linked by crossveins, to form 

 patterns such as those that are well shown in Figs. 30, 31. 



The arrangement of these veins is not haphazard, as artists some- 

 times show it. On the contrary in nearly all groups of insects the 

 wing-venation^ as it is called, is remarkably constant, and can be used 

 to distinguish one group from another. There are a number of systems 

 of names or numbers for the veins, and for the cells, or spaces en- 

 closed by the veins. The Comstock-Needham system of wing-venation 

 was intended to apply to all groups, in the belief that they could be 

 derived from one ancestral form. Space does not permit an account of 

 this system here, but it will be found in textbooks of entomology. 



The operation of the wings in flight is discussed in Chapter V. 



The abdomen shows the original segmentation more clearly than any 

 other part of the insect's body. Each segment normally has an upper, 

 or dorsal plate, or tergum, and a lower, or ventral plate, or sternum, 

 joined at the sides by a soft membrane, which allows the abdomen to 

 expand and contract. The value of this flexibihty is seen in the move- 

 ments of the abdomen during breathing; when taking in Hquid food, as 

 when the abdomen of a mosquito swells with blood; when an insect 

 stores food for the winter by enlarging its fat-body; and when the 

 abdomen of a female insect becomes distended with eggs. 



Generally the main part of the abdomen is made up of seven seg- 

 ments, though this number may be reduced if one or more segments 

 have been lost, or have fused together. These segments rarely have any 



29 



