2 PHYLUM ARTHROPODA 



Observe that the body of the animal is composed of a number 

 of serially arranged segments. These are called somites, or meta- 

 m_ereSj and the segmented type of structure presented by the in- 

 sect body is called a m-etameric type of structure. Observe that 

 the body is sharply divided into three divisions — the head, 

 thorax, and abdomen. 



The head is unsegmented and bears on its anterior and dorsal 

 surface a pair of long, jointed feelers, or antennae, which are im- 

 portant sense organs, a pair of large compound eyes, and three 

 small, dotlike eyes, called ocelli, which it may be necessary to 

 look for with a hand lens ; on its ventral side are the mouth parts, 

 the organs which taste, grasp, and masticate the food. Examine 

 these mouth parts carefully with a hand lens ; notice that there 

 is a short, overhanging upper lip, beneath which is a pair of power- 

 ful jaws having a lateral, or side, position instead of a dorso- 

 ventral one like the jaws of vertebrates. Beneath the jaws are 

 two other pairs of mouth parts, the maxillae and the under lip, 

 which, however, will not be studied at present ; notice the two 

 pairs of elongated and segmented palps, which are probably or- 

 gans of taste. 



The thorax is composed of three somites, or metameres, which 

 are called, respectively, the prothorax, the mesothorax, and the 

 metathorax. Each somite bears a pair of legs on its ventral sur- 

 face, and the mesothorax and metathorax bear each a pair of 

 wings on the dorsal surface ; thus the organs of locomotion of 

 the animal are concentrated in the thorax. Find the sutures 

 between the thoracic segments. The dorsal cuticula of each 

 thoracic segment is called the tergum; the ventral cuticula, 

 the sternum ; and that of each lateral side, the pleurum. Thus 

 we speak of the presternum, the mesosternum, and the meta- 

 sternum, etc. 



In the abdomen the dorsal and the ventral portions of the 

 cuticula are composed each of a distinct plate in each somite, 

 which are called the tergite and the sternite, respectively. The 

 abdomen bears no appendages ; it contains most of the vegeta- 

 tive organs of the animal. At its hinder end are the vent, or anus, 

 and, in the female, the sting. Look with the aid of a hand lens 



