INSECTA. 



193 



and legs in addition to those which previously or originally existed ; 

 these Insects are therefore termed Myriapoda. 



True or hexapod Insects have thirteen rings, one for the head, three 

 for the thorax, and nine for the abdomen. Certain flying Insects 

 in their early or larval state present several pairs of rudimental feet, 

 in addition to those attached to the first three segments, succeeding 

 the head, but no true Insect in its mature state has more than the 

 three pairs of articulated limbs just indicated. 



Every Insect has a distinct head {Jig- 96, a), provided with one 



pair of antennae (c), and its 

 trunk is divided into two 

 regions called thorax and 

 abdomen (a b). 



The thorax is interposed 

 between the head and the 

 abdomen, and so far is ana- 

 logous to that part in hu- 

 man anatomy ; but it has 

 neither the same relation to 

 the contained viscera nor 

 to the locomotive extre- 

 mities which characterise 

 the thorax in the vertebrate 

 animals. To the Insect's 

 thorax are attached all the 

 locomotive members ; both 

 the first pair, which may 

 be compared w ith the pec- 

 toral extremities of the ver- 

 tebrate animal, and the last 

 pair, which may corre- 

 spond with the pelvic members, as well as the middle pair, to which 

 there is no analogue in the vertebrate series. This centre of the 

 locomotive powers is divided into three segments, which correspond 

 with the three pairs of legs : the first segment is termed the " pro- 

 thorax " (c?), the second the " meso-thorax " (/), and the third, 

 the "^ meta-thorax " (J,). Each of these segments has a dorsal and a 

 sternal piece : the dorsal half rings are called respectively " prono- 

 tum," " mesonotum," and " metanotum ; " the ventral or sternal arcs 

 bear the corresponding terms, " presternum," " mesosternum," and 

 " meta-sternum." From the inferior arches of the segments, the 

 legs (e, h, k) are developed, or with them they are principally arti- 

 culated, like the legs of the Crustacea and the ventral oars or seti- 



o 



Locusta. 



