268 The Orders of Insects 



degraded of the Diptera. The small head is turned upwards and 

 backwards into a cavity on the thorax, which is round or oval and 

 bears on either side a comb-shaped organ, probably a vestigial wing, 

 and a minute balancer. The legs are long with flattened shins and 

 five-segmented clawed feet. The few known species live as parasites 

 on bats. 



ORDER 15.— HYMENOPTERA. 



Structure. — The Saw-flies, Gall-flies, Ichneu- 

 mon and other parasitic flies. Ants, Wasps and Bees, 

 which are included in this Order, are characterised 

 by their two pairs of membranous wings with com- 

 paratively few nervures and cells, well-developed 

 mandibles, and very highly specialised body-structure. 

 Their most remarkable feature, perhaps, is the partial 

 or entire fusion of the first abdominal segment {propo- 

 deum) (fig. 18 «) with the thorax; the "waist" which 

 is so marked a feature in many Hymenoptera comes 

 therefore not between thorax and hind-body but 

 behind the first abdominal segment. The dorsal 

 region of the prothorax is much reduced but the 

 pronotum (fig. 18^) is largely developed at the sides 

 and even towards the lower aspect of the body, the 

 prosternal sclerites (fig. 18 a, /^) being correspondingly 

 reduced, and in some cases quite hidden. The meso- 

 thorax (fig. 18 e,f, g, h, i,j, k) forms the greater part 

 of the fore-body ; at the base of each wing is a small 

 plate — the tegiila (fig. 18 /). As the apparent thorax 

 contains, as mentioned above, an abdominal segment, 

 it is sometimes known as the "alitrunk." The six legs 

 are always well-developed, and the feet are usually 

 five-segmented ; in some families each trochanter has 

 two segments. The wings are usually rather narrow, 

 and the fore-wing is always longer and broader than 

 the hind -wing \ the costa of the latter is, in most 

 cases provided with a row of curved hooks which 

 fasten on to a fold in the dorsum of the fore-wing. 



