HOLOMETABOLIC INSECTS. 599 



Division C. Holometabolic Insects. 



Order I. Aphaniptera. — Whigs rudimentary, in the form of 

 scales situated on the mesothorax and metathorax. Mouth suctorial. 

 Metamorphosis complete. 



This order includes only the Fleas, and requires no further con- 

 sideration here, since no fossil representatives of the group are known 

 to exist. 



Order II. Diptera. — This order includes the "Two-winged 

 Flies," and is characterised by the fact that the mouth is suctorial 

 and is also adapted for piercing ; the anterior wings are membranous, 

 with a small basal lobe (" alula "), not closely reticulated, but with a 

 few, definitely-placed cross-veins. The posterior wings are rudimentary, 

 and are represented by a pair of clubbed filaments (" halteres"). 



As regards the distribution of the Diptera in time, a few repre- 

 sentatives of the order have been detected in rocks as old as the 

 Jurassic, but the majority of the fossil forms are found in Tertiary 

 deposits, being especially abundant in amber. Most of the existing 

 families of Diptera are represented by fossil forms, but none of these 

 present any points of special interest. 



Order III. Lepidoptera. — This order includes the Butterflies 

 and Moths, and is characterised by the fact that the mouth of the 

 adult is completely adapted for suction, the mandibles being rudiment- 

 ary. Both pairs of wings are p?'esent, and are membranous, being 

 covered with rows of flattened hairs or scales. The neuration of the 

 wings (fig. 442, b) is simple, the nervures being rarely united by cross- 

 veins. The "marginal" vein is wanting; while the "scapular" and 

 " externomedian " veins generally form between them a median cell, 

 and give origin to most of the secondary nervures. The metamorphosis 

 is complete, the larva ^caterpillar") being worm-like, and the pupa 

 (" chrysalis ") being completely enclosed in a horny integument. 



As regards their distribution in time, undoubted remains of 

 several species of Lepidoptera (such as the Sphinx Snelleni of the 

 Solenhofen Slates) have been detected in the Jurassic rocks ; and 

 the tunnels of the larvae of small Moths have been noticed in fossil 

 leaves of Cretaceous age ; but the number of recognised Mesozoic 

 Lepidoptera is very limited. In the Tertiary rocks the remains of 

 Lepidoptera are still exceedingly rare, but both the Moths and the 

 Butterflies are now represented by fossil forms. About a dozen 

 Tertiary species of Butterflies are known altogether, and most of 

 these belong to extinct genera (Mylothrites, Neorinopsis, &c.) 



Order IV. Hymenoptera. — This order includes the Ants, Bees 

 and Wasps, Gall-flies, &c, and is characterised by the fact that the 

 mouth-organs are adapted partly for suction and partly for biting, the 

 mandibles being well developed. Both pairs of wings are usually 



