654 CLASS IV. INSECTA. 



are well represented. The best preserved specimens are found 

 in the amber of the Oligocene in mid-Europe ; in this speci- 

 mens allied to Lepisma occur. Earwigs first appear in the Lias, 

 cockroaches in the Trias and generalized locusts and grasshoppers 

 in the coal-measures. This formation also contains a number of 

 forms of Termites, Ephemerids, Dragon-flies. Beetles are found 

 first in the Trias and become common in the Jurassic and 

 Tertiary beds, in which the Cicadidae and Notonectidae are also 

 met with. Butterflies and moths are found in the Jurassic 

 strata but are amongst the rarest of Insect fossils. A few Diptera 

 are found in the Lias and in the Oolitic Solenhofen Limestone, 

 they become more numerous in the Tertiary strata and the 

 same is true of the Hymenoptera.* 



Classification. Insects are classified mainly on three features : 

 (i) the absence, presence and the nature of their wings ; (ii) the 

 structure of their mouth parts ; and (iii) the degree in which they 

 undergo metamorphosis. | In the following account of the 

 classification the large groups are in the main based on 

 wing- characters, but these run to some extent on lines parallel 

 with those laid down by our knowledge of the metamorphosis 

 of the insects concerned. 



There are four main groups : J 



I. APTERYGOTA. 



Wingless insects whose ancestors are believed to have been 

 also wingless. Metamorphosis very slight. This group includes 

 but two Orders, the (1) COLLEMBOLA and the (2) THYSANURA. 



II. ANAPTERYGOTA. 



Wingless insects supposed to have developed from winged 

 ancestors. They are all parasitic on vertebrate animals. This 

 group includes the Orders (3) MALLOPHAGA and (4) ANOPLURA 

 in which the metamorphosis is very incomplete and the 

 (5) SIPHON APT ERA in which the development is rapid and the 

 metamorphosis complete. 



III. EXOPTERYGOTA. 



Winged insects whose wings develop outside the body and 



* H. Woods, Palaeontology, Invertebrata, 1902. 



f Instar is the term applied to the form of an insect between two moults 

 (Sharp, op. cit., i, p. 158). 



J D. Sharp, Proc. International Congress of Zoology, Cambridge, 1898. 



