INSECTA 461 



easily linked together by intermediate forms and the story of evolution 

 within the subphylum consists rather of disjointed sentences than 

 a continuous theme. The two divisions already mentioned, however, 

 the Exopterygota and Endopterygota, are natural groups which we 

 may for convenience call the "generalized" and the "specialized" 

 respectively. The former have for the most part biting mouth parts 

 (the Hemiptera forming an important exception), while the latter 

 have their mouth parts modified in many remarkable ways enabling 

 them to tap sources of food forbidden to the others, such as the in- 

 ternal fluids of plants and animals and the deeply hidden nectar of 

 modern flowering plants. Moreover, the life cycle in these two divi- 

 sions is very difl^erent, the exopterygote (hemimetabolous) insects 

 having a gradual metamorphosis with external wing growth and the 

 endopterygote (holometabolous) forms having a complex meta- 

 morphosis with internal wing growth and a pupal stage intercalated 

 in the life history to bridge the gulf between dissimilar larvae and 

 adults. 



From a morphological study alone one is driven to the conclusion 

 that the insects with biting mouth parts and simple metamorphosis 

 are the most primitive — i.e. more nearly resembling the ancestral 

 forms than the Endopterygota. It is of great interest therefore to 

 find that the palaeontological record, though discontinuous, supports 

 the conclusions drawn from comparative anatomical investigations. 



The first records of insects are to be found in rocks of the Devonian 

 period. Here they consist of remains which, though fragmentary, 

 suggest that wingless insects similar to our present-day Apterygota 

 abounded then. If they were as soft-bodied as those we know to-day 

 the poverty of the record can well be understood and it is fairly certain 

 that thysanuroid insects similar to the silver fish Lepisma existed, 

 throughout the Devonian age. 



There is abundant evidence, however, that winged insects existed 

 in the Carboniferous period. There were insects with prominent 

 meso- and metathoracic wings, with lateral wing-like expansions on 

 the prothorax, and shorter pleural processes on the abdomen. The 

 order Palaeodictyoptera in which such forms have been placed has 

 given rise to much speculation as to the origin of wings, one idea being 

 that wings are hyper-developments, on the appropriate segments, of 

 lateral processes which occurred on all segments behind the head. 



In rocks of the same period have been found forms so similar to 

 our modern cockroaches that it is xlifficult not to place them in the 

 same family, mouth parts and wing venation being almost identical 

 in the ancient and modern types. Since such forms have existed from 

 the Carboniferous till to-day the student making his first essay into 

 the intricacies of entomology by dissecting the cockroach should keep 



