Chap, ix.] APPENDAGES OF INSECTS. 303 



pair of antennae retain the natatory function which 

 they have in the Nauplius stage. In the Ciriipeclia 

 the six pairs of appendages behind the gnathites have 

 the exopodite and endopodite consisting of a large 

 number of joints, and they form the filamentous cirri 

 which are so characteristic of these animals. In the 

 Arachnida there are no appendages in front of the 

 head, antennae being absent. In Peripatus the an- 

 tennae do not belong to the series of ventral appendages 

 of the segments of the body. In Myriopods and In- 

 sects there is a single pair of antennas. In the 

 Arachnida the basal parts of the circum-onil appen- 

 dages alone take part in the service of the mouth ; 

 the most anterior pair are pincer-shaped at their free 

 end (cholicersc) ; in some Myriopods one of the 

 anterior pairs of appendages become poison- claws, as 

 are the chelicerse in spiders. In the parasitic Pen- 

 tastomida all signs of appendages are reduced to two 

 pairs of curved hooks in the region of the mouth. 



The Hexapoda have only three pairs of gnathites 

 (see page 128) ; and these, as has been already pointed 

 out, present the most diverse modifications in different 

 orders. The legs are almost always well developed on 

 the three segments of the thorax, and are typically 

 five-jointed ; the most proximal is known as the coxa- 

 and this is succeeded by the ordinarily smaller 

 trochanter, by the longer femur, and the still longer 

 tibia; the last joint or tarsus consists of several 

 pieces, the most distal of which, or that farthest from 

 the axis of the body, carries a pair of claws. 



The number of legs (six) must be a great me- 

 chanical advantage to an insect, for three supports are 

 necessary to maintain a stable equilibrium. They may 

 become adapted to very various modes of progression 

 through earth or air. In the digging forms, such as 

 Gryllotalpa (the mole-cricket), the tibiae of the first 

 pair of legs are flattened, triangular, and toothed, 



