236 Arthropoda. 



the thoraXj and are moved by a muscular apparatus ; they are 

 usually thin transparent plates, in which a network of somewhat 

 thicker, more firmly chitinised, and darker ribs occur : not in- 

 frequently they are sparsely or entirely covered with setee {see 

 the Lepidoptera) . The two pairs are often almost identical in form 

 and size, more frequently, as in certain Libellulidfe, they differ 

 somewhat in these respects ; sometimes the anterior, sometimes the 

 posterior, pair is the larger. During flight, they are spread laterally, 

 but when at rest, they turn somewhat backwards, so as to cover 

 the abdomen, and the wings of the first pair overlie the second 

 ones, which are then often folded like a fan.* In correlation with 

 this, the anterior wings have been modified, in many Insects (Locusts, 

 Beetles), to form wing-cases, or elytra; they are thicker and 

 harder, and serve chiefly or exclusively to cover and to protect the 

 posterior pair during rest, whilst their locomotor importance is 

 lost ; the hind wings, which are usually large, lie beneath them, 

 folded longitudinally or transversely. The elytra attain their greatest 

 development in Beetles (Fig. 197), where they not only protect the 

 posterior wings most eflBciently, but also the dorsal surface of the 

 abdomen (which is, therefore, softer than the ventral side) ; for 

 their inner edges are straight, and fit closely together, and their 

 outer edges are coincident with the lateral body-wall. It results, 

 therefore, that in many Beetles, which are apterous, or have rudi- 

 mentary wings only, well-developed elytra are, nevertheless, present. 

 Another modification of one pair of wings occurs in the Diptera, where 

 the hind ones are developed as small club-like appendages 

 (h a 1 1 e r e s) , the significance of which is not clear, but which are 

 certainly not organs of flight. In a number of Insects belonging to- 

 various groups, the wings are rudimentary, or altogether absent ; 

 many of these forms are parasitic. 



The abdomen, the posterior apodous region of the body, consists- 

 of ten or fewer segments, which are usually freely articulated, although 

 occasionally some of them are fused ; there is not generally such a 

 deep constriction between the thorax and abdomen as between the 

 head and thorax. In each abdominal segment a dorsal and a ventral 

 plate (tergum and sternum) is usually distinguishable, connected by 

 softer portions. In some Insects (Mole-cricket, Dragon-fly), the 

 posterior end of the abdomen bears a pair of jointed or unjointed 

 anal cerci, which turn backwards, but otherwise there are no- 

 abdominal limbs or limb-like appendages. t 



* But occasionally the first pair of wings are folded when at rest. 



t In some genera belonging to the Thysanura, a group which consists entirely 

 of apterous Insects with biting mouth parts, there are, on the ventral side of the- 

 abdominal segments, small paired appendages, which are not jointed, but which are 

 quite like limbs in their mode of origin. It must be mentioned, too, that in many 

 insectan embryos, definite rudiments of limbs bud out from the first abdominal, 

 segments (sometimes from several), but these atrophy before hatching. 



