170 INJURIOUS AND USEFUL INSECTS 



body, becoming longer at each successive moult; in an old 

 larva the fore wings are particularly evident, over-arching the 

 fore part of the abdomen. 



It is to be remarked that the fresh - hatched larva is of 

 different form, wanting the gills and the central tail-filament. 

 Similarly, a fresh-hatched Chironomus-larva differs from old 

 ones in having no blood-gills, and no red colouring-matter in 

 its blood (see p. 125). 



A year after the beginning of the larval stage the imaginal 

 development is completed, and the insect quits the water, leav- 

 ing behind it only an empty larval skin. Some Ephemerids, 

 which inhabit muddy rivers in vast multitudes, hatch out 

 so accurately at the same time every year that their advent 

 can be noted in a calendar. They emerge towards night, and 

 fill the air, like the flakes in a snowstorm. I have seen the 

 streets covered with them the day after emergence, and the 

 heaps into which they were swept lying piled about every lamp- 

 post. The plague, for such it is in certain places, lasts two or 

 three nights, and is then over. Our common May-flies are 

 less obtrusive ; they come out night after night for perhaps a 

 fortnight, at midsummer or a little earlier, and congregate in 

 swarms of no great size, consisting usually of males only. 

 The females quit the swarm as soon as they are fertilised, 

 and make their way to the water in order to lay their eggs, 

 while the polygamous males return to the swarm. The same 

 habit is to be remarked in many gnat-like Diptera (see p. 119). 



A very mysterious peculiarity of Ephemerids is that the flies 

 leave the water, clad in a thin, transparent, close-fitting pellicle, 

 like a pupa - skin ; this may subsequently be shed, either 

 partially or wholly, but in some cases the fly mates and dies 

 without becoming free. 



The winged Ephemerid is a slightly - built fly, with ample 

 gauzy wings, the fore pair much larger than the hind, which 

 may disappear altogether (Chloeon dipterum). There are two 

 or three long tail-filaments in most of the species. The legs 

 are slender ; in the male fly the fore legs are often longer than 

 the rest, erected in the air, and used only to grasp the female. 

 The antennae are short, and the mouth-parts reduced to such 

 a point that the fly is incapable of feeding. The abdomen 

 looks plump, as if well-filled, but it is merely distended with 

 eggs (if a female) or with air. Male flies are distinguished by 



