744 



THE HEX A POD A 



The exoskeleton of the head is composed of sclerites — ^that is to say, more 

 densely chitinized regions of the integument— which are: (i) The clypeus; 

 (2) the epicranium; (3) the gula. 



The clypeus is the sclerite situate on the anterior portion of the dorsal sur- 

 face, and carrying the labrum; in flies it is often called the face. 



The epicranium is the larger part of the head, and may be subdivided 

 into an anterior frons and a posterior occiput. The genae form the sides of 

 the head, meeting the epicranium and the gula at the occipital foramen. 



The gula is the sclerite in the median ventral line which carries the basal 

 part of the labium called the submentum. There is no exoskeleton at the 

 posterior part of the head ventral to the occipital region, which thus forms a 

 foramen in the hard tissues called the occipital foramen, through which the 

 soft structures of the head communicate with those of the thorax. 



The antennae appear to be sensory organs, and the mandibles to vary in 

 structure according as to whether the mouth is to be used for biting or sucking. 

 In the former case they are broad and strong, while in the latter they are 

 styliform. The first pair of maxillae, similarly, may be broad, strong organs or 

 styliform organs. 



Fig, 364. — Wing of a Mosquito to illustrate the Venation. 

 (From a photograph by J. J. Bell.) 



In addition to these appendages, the mouth shows an upper lip or labrum, 

 which is simply a sclerite attached to the cephalic shield, and may have a 

 median projection from its internal surface called the epipharynx, while the 

 labium has a similar one called the hypopharynx. Thus the mouth parts of 

 an insect may be very complicated, with labrum and epipharynx, mandibles, 

 maxillae, and maxillary palps, labium, and hypopharynx. 



The thorax is joined to the head by a neck, and is subdivided into three 

 segments — prothorax, mesothorax, and metathorax. These somites are by 

 no means simple horny rings, but have their chitinous exoskeleton split up 

 into hard pieces joined together by soft material. The hard pieces are a dorsal 

 plate called ' the notum,' a ventral plate called ' the sternum,' and la,teral 

 plates called ' pleura.' Further, the terms, ' pro,' ' meso,' and ' meta,' are 

 applied to these, indicating the region to which they belong — ' pronotum,' 

 ' mesonotum,' and ' metanotum '; ' pro-,' ' meso-,' and ' metasternum ' — 

 while each pleuron is divided into an anterior episternum and a posterior 

 epimeron. According to Audouin, a typical thoracic segment should have 

 a notum composed of praescutum, scutum, scutellum, and post-scutellum, 

 but all these parts are seldom seen. 



Each somite of the thorax carries a pair of jointed walking-legs, in which 

 the segments or articles are named coxa, trochanter, femur, tibia, and tarsus 

 (consisting of several joints, the first joint of which is sometimes called the 



