GENERAL NOTES ON INSECTS. 341 



It was a step of some importance in morphology when Savigny 

 showed that the three pairs of appendages about the mouth are 

 homologous with the other appendages, i.e. are masticatory legs. 



(i) Farthest forward lie two mandibles* the biting and cutting jaws. 

 These are single-jointed, and thus differ from the organs of the same 

 name in the crayfish, which bear a three-jointed palp in addition to the 

 hard basal part. In those insects which suck and do not bite, e.g. 

 adult butterflies, the mandibles are reduced. 



(2) Next in order is theyfor/ pair of maxilla. Each maxilla consists 

 of a basal piece (protopodite), an inner fork (endopodite), and an outer 

 fork (exopodite). The entomologists divide the protopodite into a 

 lower joint, the cardo, and an upper, the stipes ; the endopodite into an 

 internal lacinia and an external galea ; while the exopodite is called 

 the maxillary palp. 



(3) The last pair of oral appendages or second maxilla are partially 

 fused, and form what is called the labium. The lower and upper 

 joints of their fused protopodites are called submentum and mentum ; 

 the endopodites on each side are double, as in the first maxillae, and 

 consist of internal lacinia and external paraglossa ; the exopodites are 

 called the labial palps. 



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 Gollembola 

 and Thysanura, ever had wings. 



There are many interesting differences 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 flies 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 ; butter- 

 flies 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. 

 Tt 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. 



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