TEXT-BOOK OF ZOOLOGY 



stroke, the insect rises ; if, on the other hand, the upward stroke is the 

 stronger, it sinks downwards towards the ground. The direction of the 

 flight also depends on the position in which the abdomen is placed (see 

 Part II., p. 151, Section d), and the direction in which the stroke of the 

 wing is executed. (If the wing is struck vertically, the animal flies in a 

 straightforward direction ; if horizontally, it will rise or fall.) Deviations 

 to right and left are accomplished (primarily) by movements of the abdo- 

 men, which acts exactly in the manner of a rudder. 



3. Digestion. (a) In all insects three pairs of the limbs (see p. 308, 

 Section 2) enter into the service of the nutritive function. The first pair 

 form the unsegmented biting jaws, or mandibles (Ok.) ; they are situated 

 below a plate, the labium, or upper lip (01.), which represents a prolonga- 

 tion of the anterior portion 

 of the chitinous capsule of 

 the head. Below the man- 

 dibles are placed the maxillce 

 (Uk., second pair). They 

 consist of several joints, and 

 carry feeler-like processes, 

 the maxillary palpi (Kt.). 

 The most inferiorly placed 

 pair of masticatory limbs, 

 viz., the third, as a rule 

 coalesce (see, however, Li- 

 bellula) into an apparently 

 single piece, the lower lip, 

 or labium (plj, and are also 

 furnished with a pair of labial 

 palpi (Lt.). The shape and 

 structure of all these parts" 

 vary considerably, according 

 to the different nature of the 



food^in the several insect orders (give examples). The palpi, which are 

 provided with organs of smell and taste, are probably used for the pur- 

 pose of testing the food. 



(b) As in vertebrates, digestion takes place in the intestine, which 

 usually consists of several separate divisions (see, e.g., the honey-bee). 

 Two large salivary glands always open into the mouth. 



(c) Kidneys are represented by tubular vessels, which are very long, 

 and in many cases very numerous. They are termed Malpighian vessels, 

 after Malpighi, a mediaeval Italian naturalist. 



4. Respiration. (a) Insects, living as they do with few exceptions 



MOUTH PARTS OF A BEETLE (FROM BELOW). 

 K., Masticatory portion of the maxilla ; Z., middle 

 and anterior segment of the labium, or "lingua"; 

 A., eye; F., antennae. The other letters are ex- 

 plained in the text. 



