WASPS 199 



upturned end (propodeum) ; the pressure will make the thorax 

 shorter but deeper, and will depress the wings ; (2) place the 

 thorax in its natural attitude, and press upon its highest point ; 

 this will render the thorax longer but shallower, and produce 

 elevation of the wings. 



The three pairs of legs are all of similar structure, and differ 

 but slightly in length ; the anterior being the shortest, the 

 posterior longest. Each leg is composed of nine joints : the 

 part next to the thorax (coxa) is short and flattened ; the next 

 (trochanter) is very small, is sunk into its articulations, and thus 

 affords a degree of freedom of movement approximating to that 

 obtained by a ball-and-socket joint ; this is followed by the 

 thigh (femur), a long and rather swollen portion standing almost 

 horizontally at right angles to the body, then the shin (tibia), 

 which is straight and about the same length as the thigh, and 

 lastly the ankle (tarsus), composed of five joints, and terminating 

 in two curved claws. The limbs are virtually hollow, jointed 

 tubes, the articulations between the successive portions being 

 formed merely by thinner and therefore more flexible bands 

 of the ordinary chitinous exo-skeleton. The cavities of the 

 limbs are occupied by muscles which pass from one joint to the 

 next through the articular ring, and which by contracting bend 

 one joint upon the next in this or that direction. By appropriate 

 bending or extension of the successive joints the foot can be 

 moved in almost any direction. When the insect walks the first 

 pair of legs pulls the body forward ; the second pair partly pulls 

 and partly pushes, the former process at the beginning and the 

 latter at the end of a " step," but is chiefly for the purpose of 

 balancing and sustaining the body ; the third pair pushes the 

 body onwards. The legs are moved in the following order. 

 The first and third of the right side move forwards at the same 

 moment as the second of the left, and then the first and third 

 of the left and the second of the right are simultaneously ad- 

 vanced. Thus the animal is always supported on at least three 

 points. In fact, its walking apparatus might be termed a pair 

 of tripods, used alternately. The claws in which the legs ter- 

 minate give a secure hold upon chance inequalities of the path 

 travelled over, and between them is a pad which secretes a sticky 



