404 THE INVERTEBRATA 



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 different, 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 diflicult 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 flrst essay into 

 the intricacies of entomology by dissecting the cockroach should 

 keep in mind that he is dealing with a very ancient type — a real 

 aristocrat among insect species ! 



In both the Ephemeroptera and Odonata we find many generalized 

 characters — in the mouth parts and the reticulate wing venation — 

 and these orders had their origin in the Permian, when forms assigned 

 to the two orders Protephemeroptera and Protodonata abounded. 



