MOUTH-PARTS, WINGS, AND LEGS 45 



very large, extending right down into the abdomen, and 

 the flow of saliva is proportionately copious. Thus, when 

 the fly wishes to feed — upon a lump of sugar let us say — 

 it simply applies the tip of its labium to the substance, 

 floods the sixty channels with saliva, and then sucks this 

 back again together with the sugar which has been dis- 

 solved. Between these two extremes — the gnat and the 

 house-fly — almost every conceivable adaptation of the 

 mouth-parts for piercing and sucking may be found among 

 two-winged flies ; while in the case of the bot-flies the 

 mouth-parts are practically obsolete, and the adult insect 

 takes no food. Yet all these marvellous changes are 

 believed to have been effected by twisting, lengthening, 

 compressing, remodelling, or suppressing the three pairs 

 of ancestral foot-jaws with which insects were originally 

 endowed. 



Next to the mouth-parts of an insect the wings are 

 its most characteristic feature. Indeed, if we find out how 

 a given insect eats, and what kind of wings it possesses, 

 we can usually interpret its natural affinity. We have 

 already seen that the typical insect has two pairs of wings, 

 that these spring from the second and third segments of 

 the thorax, and that they consist of a twofold layer of 

 skin. Each wing is supported by longitudinal veins, or 

 nervines, which are often connected by cross nervules, the 

 number and arrangement of which are so constant in the 

 same kind of insect that an expert can usually refer a de- 

 tached wing to its correct genus, and often to its species. 



In some insects, notably dragon-flies, the two pairs of 

 wings are alike in size and form ; but more commonly 

 the fore-wings are both larger and broader than the hind- 

 wings, as in the case of may-flies, bees, and wasps. Some- 

 times, however, the fore-wings are relatively narrow, more 



