PHYLUM ARTHROPOD A 241 



Somites of the Head. — As in the other arthropods, paired 

 appendages are considered as a criterion for the determination of 

 the number of somites in the head. There are evidences that the 

 eyes, antennae, mandibles, maxillae, and labium are borne on 

 distinct segments. Beyond this, study of early embryological 

 stages furnishes evidence that an additional pair of appendages 

 starts to form in the embryo but never becomes functional or is 

 at most rudimentary in the adult. Traces of this embryonic 

 appendage are between the fundaments of the antennae and 

 mandibles. Thus in position these correspond to the second 

 antennae of the crayfish and may be homologized with them. 

 It thus seems probable that the head of an insect has resulted 

 from the fusion of six original somites only five of which have 

 retained their appendages or their rudiments in the adult insect. 



The Thorax. — The thorax is the region which bears the legs and 

 wings when they are present in the nymphs and adult insects. It 

 is composed of three more or less firmly united segments. In 

 order, backward from the head, these are: prothorax, mesothorax, 

 and metathorax. In many insects, the last two bear wings. 

 Each somite is composed of several sclerite groups which, accord- 

 ing to their location, are recognized as comprising the parts of the 

 tergum (dorsal wall), sternum (ventral wall), and pleura (lateral 

 walls) . 



The Legs. — Each leg is articulated to the wall of the thorax 

 partly by means of small articular sclerites in the region of articu- 

 lation of the sternum and pleuron. Five divisions or regions are 

 recognizable in each leg. From articulation outward, these are: 

 coxa, trochanter, femur, tibia, and tarsus. In most insects, each 

 region is but a single segment except the tarsus which commonly 

 has five segments. Correlated with highly variable modifications 

 in function the legs display numerous modifications in form. 



The Wings. — In many of the winged insects the mesothorax 

 and the metathorax each bears a pair of wings, but in instances of 

 only one pair of wings these are usually borne upon the meso- 

 thorax. Rudiments of the second pair of wings are frequently 

 present as halteres or balancing organs upon the metathorax. 

 Some striking modifications of the primitively membranous 

 wings occur. In beetles, the mesothoracic wings are thickened, 

 horny structures, the elytra, modified for the protection of the 

 metathoracic wings and the dorsal surface of the body. In some 

 of the bugs, the bases of the front wings (hemelytra) are horny 



