18 



PARASITES OF THE DOMESTIC ANIMALS 



the blood sinuses and tracheae. Sometmies the anterior, sometimes the 

 posterior pair is the larger, and both may be flexible and adapted for 

 flight. In some insects (beetles) the anterior pair is modified to form 

 wing-shields, or elytra, which are hard, but 

 slightly flexible, structures serving to cover 

 and protect the posterior wings during rest. 

 Some insects possess but one pair of wings 

 (dipterous), while in others wings are entirely 

 absent (apterous). 



The abdomen is segmented, the number of 

 segments varying with different groups. Each 

 segment consists of two cuticular plates (Fig. 6), 

 the dorsal tergite and the ventral sternite, 

 which are united laterally by a softer mem- 

 brane, the pleurite. There are no abdominal 

 limbs or limb-like appendages. 



Respiration is by tracheae (Fig. 5), a system 

 of tubes containing air. These communicate 

 with the outside by 

 the spiracles (Fig. 

 6), small s>anmet- 

 r i c a 1 1 y disposed 

 openings located 

 laterally, one pair 

 on the meso- and 

 one pair on the 

 metathorax and a 

 pair on each of the 

 abdominal segments 

 except the most posterior. Just inside of the spiracles the tracheae are 

 usually united by longitudinal trunks from which are given off fine 

 branches which ramify and anastomose within the body. Respiration 

 is effected by abdominal movements of contraction and expansion. 



Insects are mostly oviparous. In some the developed embryo is 

 released from the egg while still within the body of the parent, or this 

 may occur just as the egg is extruded. There are also pupiparous forms 

 where the young pass from the body of the female ready to enter the 

 pupal stage in their development. 



In order that the newly hatched larvae may be supplied with nourish- 

 ment, the eggs are generally deposited where suitable food is present. 

 In many insects oviposition occurs by means of an ovipositor, a tube- 

 like organ which is developed from the posterior abdominal segments 

 and which may project free from the body or may be retracted into it. 

 In the Hjmienoptera the ovipositor may be modified to serve as a sting, 



Fig. 5. — Diagram show- 

 ing the chief trunks of the 

 tracheal system of an in- 

 sect (after Boas, by Kirk- 

 aldy & Pollard). 



Fig . 6. — Abdomen of Lo- 

 cust, s howing Spiracles 1, 2,13, 

 4, 5, 6, 7 and 8, one on each side 

 of each of the abdominal seg- 

 ments; A, auditory sac (drawn 

 in part from Packard's Zool- 

 ogy). 



