334 COLLEGE ZOOLOGY 



(M), cubitus (Cu), and anal (A). Cross veins (7, /, K) 

 frequently occur. Modifications come about by reduction 

 or by addition. In the beetles (Coleoptera) the fore-wings 

 are sheath-like, and are called elytra. The fore-wings of 

 Orthoptera (grasshoppers, etc.) are leathery and are known 

 as tegmina. 



Of the internal organs of insects the alimentary canal and res- 

 piratory systems are of particular interest. The alimentary 

 canal is modified according to the character of the food. An 

 insect with mandibulate mouth-parts (Fig. 256) usually pos- 

 sesses (1) an oesophagus (Oe) which is dilated to form a crop (Jn) 

 in which food is stored, (2) a muscular gizzard or proventriculus 

 (Pv) which strains the food and may aid in crushing it, (3) a 

 stomach or ventriculus (Chd) into which a number of glandular 

 tubes (gastric coeca) pour digestive fluids, and (4) an intestine (R) 

 with urinary or malpighian tubules (Mg) at the anterior end. 

 Suctorial insects, like the butterflies and moths (Fig. 257), are 

 provided with a muscular pharynx which acts as a pumping organ 

 and a sac ( V) for the storage of juices. 



The respiratory system of insects is in general like that of the 

 honey-bee (p. 320, Figs. 242 and 243), but modifications occur in 

 many species, especially in the larvae of those that live in water. 

 Aquatic larvae, in many cases, do not have spiracles, but get 

 oxygen by means of thread-like or leaf-like cuticular outgrowths 

 at the sides or posterior end of the body, termed tracheal gills 

 (Fig. 261, A). Damsel-fly larvae possess caudal tracheal gills, 

 and the larvae of the dragon- flies take water into the rectum 

 which is lined with papillae abundantly supplied with tracheae. 

 The economic importance of a tracheal respiratory system has 

 already been pointed out (p. 332). 



Growth and Metamorphosis. — - Three types of insects may 

 be distinguished with respect to the method of their develop- 

 ment, (1) ametabola, (2) heterometabala, and (3) holometabola. 

 The ametabolous insects are essentially \ike the 'adult, except ir 

 size, when they hatch from the egg; they develop tojpaturitj 



