GENERAL NOTES ON INSECTS. 



325 



The three pairs of thoracic legs consist of many joints, are usually 

 clawed and hairy at their tips, and differ greatly according to their uses, 

 as may be seen by comparing, for instance, the hairy feet by aid of 

 which the fly runs up the smooth window pane, the muscular limbs of 

 grasshoppers, the lank length of those which characterise " daddy-long- 

 legs," the bees' legs with their pollen baskets, the oars of water-beetles. 



Wings. These arise as flattened hollow sacs, which grow 

 out from the two posterior segments of the thorax. They are 

 moved by muscles, and traversed by " veins " or " nervures," 

 which include air-tubes, nerves, 

 and vessel-like continuations of 

 the body cavity. Most insects 

 have two pairs, but many sluggish 

 females and parasites, like lice 

 and fleas, have lost them. On 

 the other hand, there is no 

 reason to believe that the very 

 simplest wingless insects, known 

 as Collembola and Thysanura, 

 ever had wings. 



There are many interesting differ- 

 ences in regard to wings in the various 

 orders of Insects. Thus in beetles the 

 front pair form wing-covers or elytra : 

 in the little bee parasites Strepsiptera 

 they are twisted rudiments ; in flics 

 the posterior pair are small knobbed 

 stalks (halteres or balancers) ; in bees 

 the wings on each side are hooked 

 together. When the insect is at rest 

 the wings are usually folded neatly on 

 the back ; but dragon-flies and others 

 keep them expanded ; butterflies raise 

 them like a single sail on the back ; moths keep them flat. Many 

 wings bear small scales or hairs, and are often brightly coloured. 

 It is well known that the colours also vary with sex, climate, 

 and surroundings. Most interesting are those cases in which the 

 colours of an insect harmonise exactly with those of its habitat, or make 

 it a mimetic copy of some more successfully protected neighbour. 



As to the origin of wings, it may be mentioned that in many cases 

 they are of some use in respiration as well as in locomotion, and 

 the theory seems plausible that wings were originally respiratory 

 outgrowths, which by and by became useful for aerial locomotion. 

 This view is consistent with an idea, which grows in favour with 

 evolutionists, that new organs develop by the predominance of some 

 new function in organs which had some prior significance. Moreover, 



FIG. 162. Young may-rly 

 ephemerid. After Eaton. 



Showing tracheal gills, and wings 

 appearing in front of them. 



