The diversity of insects 



develop elaborate structures in order to do something that many other 

 insects do by quite simple means. This particularly relates to courtship 

 behaviour, to the struaure of the genitalia, and to the so-called 'second- 

 ary sexual characters ', which are Hmited to one sex. We have seen how 

 the habit of gathering the males into a swarm may be a benefit to insects 

 that have a short adult Hfe, and it is obviously helpful if a male Empid 

 fly offers his mate a small ball of silk to occupy her attention, so that she 

 does not attack him during mating. It is difficult to see why so many 

 male insects have brightly coloured patches of scales on the legs, and 

 wave these, or their spotted wings, before the female in an elaborate 

 courtship; the great majority of insects manage to mate quite happily 

 without doing this. Of course the same may be said of birds, where the 

 male of the Argus Pheasant, or the Lyre Bird has resplendent plumage 

 and an elaborate dance, while the common sparrow breeds prolifically 

 without any such assistance. 



Convergence 

 Insects that live in water tend to look alike, because they all have to 

 meet the same problems of breathing and movement, and have solved 

 them in much the same way (Fig. 51). The water-bugs that swim 

 actively — the back-swimmers (Notonectidae), water-boatmen (Corixi- 

 dae) and the giant water-bugs (Belostomatidae) all have the hind-legs 

 fringed with hairs, and the tibiae are often broadened like an oar. The 

 legs are flattened and fringed in a similar way in the carnivorous water- 

 beetles of the family Dytiscidae, and to a lesser extent in the Great Silver 

 Water Beetie, Hydrophilus piceus, which is a scavenger, and a less 

 powerful swimmer than Dytiscus. 



On the other hand, the bugs and beetles that merely crawl about on 

 submerged vegetation, without swimming actively, have normal legs, 

 without any special resemblance to an oar. This is true of the water 

 scorpions (Nepidae), which are bugs, and the crawling water-beetles 

 (HalipHdae). 



Convergence is even more striking among aquatic larvae. Not only 

 the larvae of water-beetles, but the imipature stages of Ephemeroptera, 

 Plecoptera, Odonata, Megaloptera, Trichoptera and Diptera, if they 

 live submerged in water, have developed some kind of tracheal gills, 

 either arranged in pairs along the sides of the abdomen, or two or three 

 projecting from its tip. 



The water scorpion, Nepa, reaches the air by means of a long tube 

 at the tip of its abdomen. The larva of the drone fly, Eristalis, uses for 

 the same purpose a telescopic tube bearing the hind spiracles, though 

 it also has rectal gills which might obtain oxygen from the water. 



115 



