PHYLUM ARTHROPODA. INSECTS 



233 



Retina 



Vitreous body 



Pigment 



Lens 



Epidermis 



Figure 133. Ocellus or simple eye of the honey bee in longitudinal section. (Redrawn from 

 Entomology, by J.W. Folsom. Copyright 1906 by The Blakiston Company.) 



consists of three segments: an anterior 

 prothoTax, a middle mesothorax, and a pos- 

 terior metathorax. Each segment bears a pair 

 of legs; and the mesothorax and metathorax 

 each bear a pair of wings. On either side of 

 the mesothorax and metathorax is a spira- 

 cle, an opening into the respiratory system. 

 A typical segment consists of the dorsal 

 tergum composed of 4 fused sclerites in a 

 row; a lateral pleuron made up of 3 sclerites 

 on each side, and a single ventral sclerite, 

 the sternum. 



Prothorax. The saddlelike pronotum of 

 the prothorax is large and extends down on 

 either side; its 4 sclerites are indicated by 

 tranverse grooves. The sternum bears a 

 spine. 



Mesothorax. In this segment the tergum 

 is small, but the sclerites of the pleuron are 

 distinct. The sternum is large. 



Metathorax. This resembles the meso- 

 thorax. 



Legs 



Each leg (Fig. 131) consists of a longitu- 

 dinal series of segments as follows: the coxa 

 articulates with the body; then comes the 

 small trochanter fused with the femur, 

 the tibia, and the tarsus. The femora of the 

 metathoracic legs are enlarged to contain 

 the muscles used in jumping. The tarsus at 

 the end of each leg consists of three visible 

 segments; the one adjoining the tibia has 

 three pads on the ventral surface, and the 

 terminal segment bears a pair of claws be- 



tween which is a fleshy pad or lobe, the 

 pulvillus. The claws and pulvilli are used 

 in clinging to any kind of surface. 



Wings 



The wings of insects (Figs. 134 and 135) 

 arise from the region between the tergum 

 and pleuron as a double layer of epidermis, 

 which secretes the upper and lower cuticu- 

 lar surfaces. Between these are tracheae, 

 around which spaces occur, and the ciiticle 

 thickens; they (Fig. 134C) later become 

 the longitudinal wing veins. The veins are 

 of value in strengthening the wings. They 

 differ in number and arrangement in differ- 

 ent species of insects but are so constant in 

 individuals of certain species that they are 

 very useful for purposes of classification. 

 The mesothoracic wings of the grasshopper 

 are leathery and not folded; they serve as 

 covers for the metathoracic wings which lie 

 beneath them. The latter are thin and 

 folded like a fan. 



Abdomen 



The slender abdomen consists dorsally of 

 11 segments; those at the posterior extremity 

 being modified for copulation or egg laying. 

 Along the lower sides of the abdomen, there 

 are 8 pairs of small openings (spiracles) 

 through which the animal breathes. In the 

 grasshopper, the sternum of segment one is 

 fused with the metathorax; on either side of 

 this segment there is an oval tympanic mem- 

 brane covering an auditory sac. Segments 2 



