Diptera— Classes of Arthropods 36 



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fossils to help in the solution of the question. The 

 highest families of the Hymenoptera — Ants, Wasps 

 and Bees — were abundant in Miocene times and, if 

 the determination of certain obscure Lias and Oolite 

 fossils can be trusted, ants had already appeared in 

 the Secondary epoch. Remains of some lower Diptera 

 (Tipulidse, Asilidse) occur as early as the Lias, but there 

 is no certain evidence of the existence of the higher 

 families until Tertiary times (203). The wing-neuration 

 of the TipulidcC recalls that of the Panorpida and the 

 Trichoptera ; and it is possible that the primitive 

 Diptera branched off at the close of the Primary 

 epoch from the stock which, as we have already seen, 

 must have given rise to the caddis-flies and moths. 

 The origin of the Hymenoptera must probably be 

 sought farther back in the history of the metamorphic 

 insects ; their marked specialisation of body-form, 

 development and* habit, united with the retention 

 of comparatively primitive jaws suggest that they 

 must have arisen early from the old neuropteroid 

 stock. The likeness between the caterpillars of 

 moths and those of sawflies which might be supposed 

 to indicate affinity between the Hymenoptera and 

 Lepidoptera is better explained as the result of 

 adaptation to similar habits and surroundings. 



Insects and other Arthropods. — Our comparison 

 of the various Orders with one another has led us 

 back to thysanuroid forms which must have lived 

 in early Primary times, as the probable ancestors of 

 all insects. But except for the absence of wings 

 these primitive creatures showed all the leading 

 features of the Class as we know it to-day. For 

 the common origin of Insects and other Arthropods 

 we must look further back still, to a period when 

 fossil evidence fails us altogether. The problem of 

 the relationship of the Insects to other classes of their 



