'686 CLASS IV. INSECTA. 



.also to take part in respiration, since each is supplied from the 

 heart with a vessel which takes blood to the tip. The blood then 

 returns to the abdomen, where it enters the general body-cavity 

 and makes its way to the heart. Rectal respiration also occurs. 

 When ready to give rise to the winged insect the nymph 

 floats on the water, the skin splits and in a few seconds the 

 winged creature has stepped out and flown away. This instar, 

 though active and winged, is not the imago but is termed the 

 subimago : it casts a last skin and the true imago emerges to 

 live at most a few days, but oftener only for an hour or two, 

 sometimes but for a few minutes. 



May-flies, which if circumstances are favourable are born in 

 countless numbers, are a favourite food of fishes, and form 

 the " duns " " spinners " and " drakes " of the fly-fisher. 



There is but one family : 



Fam. 1. Ephemeridae. With the characters of the Order. There 

 are some two to three hundred species described, of which about forty 

 are British; but the order is little known, and many more probably exist, 

 though from the number and variety of the fossil forms it seems that the 

 group is a disappearing one. Ephemera (Fig. 433) and Cloeon are common 

 British genera. Prosopistoma is interesting, as it was long taken to be a 

 crustacean from the pro- and meso-thorax forming a gill-chamber con- 

 cealing five pairs of tracheal gills, like a Decapod. It is European and 

 African . 



Order 12. PARANEUEOPTEEA * (ODONATA : LIBELLULIDAE). 



Long, slender insects with mobile heads, large eyes and short 

 antennae ending in bristles ; four transparent wings similar in 

 >i/>pearance and size, arising behind the level of the legs, u-itlt 

 many nervures ; larvae aquatic. 



Dragon-flies are easily recognized. The head is large and a 

 great part of its surface is usually occupied by the enormous 

 compound eyes, besides which there are three ocelli. The 

 mouth parts are adapted for catching insects, which the dragon- 

 fly does on the wing. The upper Up is large, the maxillae are 

 toothed with a palp of one segment, the labiuin is wide and 

 chiefly built up of the palps, the mandibles are powerful. The 



* Calvert, Trans. Amer. ent. Soc., xx, 1893, p. 159. Evans, British 

 Libellulinae, 1845. Hagen, Ent. Annual, 1857. M'Lachlaii, Cat. Brit. 

 Neuropt. 1857, and Ent. Mon. Man,, xx, 1854, p. 251. 



