THE OPEN TYPE OF WING-GROWTH 93 



lined with cuticle, as is usual in insects. Similarly the genital 

 ducts (vasa defer cntia) of the male open separately on the ninth 

 abdominal segment. 



Mayflies are to be observed in large swarms over the surface 

 of lakes and streams. The air-filled stomach is supposed to 

 render the insects buoyant and also to facilitate the discharge 

 of the eggs by dilating and pressing against the ovaries. The 

 aerial dances of the mayfly swarms, through their short lives, 

 over the waters that serve as their breeding-places, have often 

 been described. After pairing the female drops on the water 

 her eggs in clusters, which subsequently break up so that the 

 eggs are scattered about the bottom of the stream or lake ; 

 or she rests at intervals on the surface film, allowing a few 

 eggs to fall on each occasion. From the mayfly's egg is hatched 

 a larva (Fig. 51 B) which lives in the water for a period that 

 seems excessively long when compared with the short life of 

 the winged adult, for these insects may take a year or two for 

 the completion of their transformations. The mayfly larva 

 has a firmer cuticle than its parent, which it resembles in 

 possessing a pair of tail-limbs (cerci) and six well-developed 

 thoracic legs, while its feelers are relatively much longer than 

 those of the perfect insect and often many-jointed. The 

 relatively broad head and thorax and the tapering abdomen, 

 give to the mayfly larva (Fig. 52 A C), in conjunction with the 

 conspicuous appendages just mentioned, the appearance of a 

 wingless insect belonging to the Thysanura (Figs. 96, 97) 

 or Bristle-tails. And examination of the jaws shows that 

 this likeness is not merely superficial, for the mayfly grub has 

 mandibles (Fig. 53 a) which with their elongate basal region 

 and median grinding area, resemble as those of bristle-tails 

 do the mandibles of certain Crustacea more closely than 

 those of typical insects, while attached to each side of the 

 dorsal aspect of the tongue is a relatively large and prominent 

 maxillula (Fig. 53 b). With such jaws the mayfly grub 

 masticates its food which consists largely of organic fragments 

 that maybe in the mud wherein many of these larvae live, and 

 which they swallow in considerable bulk. Others, however, 

 crawl beneath submerged stones in swift streams and prey 

 upon other insects. 



Living, as it does, submerged in water, the mayfly larva has- 



