4 MEDICAL ENTOMOLOGY 



It is only in the last two classes that blood-sucking forms occur. The 

 INSECTA are distinguished by the following special characters. The body 

 is divided into three well-marked regions, the head, thorax, and abdomen. 

 The head bears one pair of antennae, and three pairs of variously modi- 

 fied mouth appendages. There are three pairs of legs in the adult, and 

 generally two pairs of wings. The abdomen bears no appendages as a 

 rule. Metamorphosis is usually complete, the young differing markedly 

 from the adults. 



The class is divided into a number of orders, in only four of which are 

 there blood-sucking forms. These are as follows : 



DlPTERA : Flies. These are distinguished at once by 



having only one pair of wings, 

 the metathoracic pair being re- 

 presented by the halteres. The 

 larva is a maggot-like creature, 

 distinguished from the beetle 

 larva by the absence of legs. 



SiPHONAPTERA : Fleas. These are wingless insects, with 



the body laterally compressed. 

 Metamorphosis is complete, the 

 early stages resembling those of 

 the Diptera. All are blood- 

 sucking and parasitic. 



RHYNCHOTAor HEMIPTERA: Bugs. Characterized by a jointed sucto- 

 rial proboscis, which can be folded 

 under the head. The prothorax 

 is distinct from the other seg- 

 ments, and the body compressed 

 in the dorso-ventral direction in 

 those forms which are parasitic. 

 Many feed on vegetable juices. 



ANOPLURA : Lice. Small wingless insects, with a soft 



integument. The thorax is 

 indistinctly segmented, and 

 the body compressed in the 

 dorso-ventral direction. All are 

 parasitic. 



The ARACHNIDA are distinguished by having the head and thorax fused 

 into one mass, the cephalothorax ; the remainder of the body does not 

 usually consist of distinct segments. There are no antennae. The first 



