LIFE IN PRIMARY PERIOD 217 



variations did not exist in the Primary times. They only 

 began to be clearly marked, and even then in moderate 

 fashion, in the Polar regions, towards the close of the Secondary ; 

 hence there is no reason why the longevity of the larvae and 

 adult Insects of that time should not have been much greater 

 than it is to-day and have permitted them to attain a 

 greater size. 



Insects to-day grow only during their larval stage during 

 which they shed their skins three or four times, corresponding 

 to as many epochs of sudden growth. At the end of this 

 stage they shed their skins yet once more. Immediately 

 after this last sloughing, short, oval sheaths, the rudiments of 

 the wings, are seen attached to their meso- and metathorax. 

 After this sloughing they either preserve their activity, as in 

 the case of the Neuroptera, Orthoptera, and Hemiptera, 

 the only orders represented in Primary times, or lose the power 

 of moving their cephalic and thoracic appendages, as in the 

 case of the more recent Coleoptera, Hymenoptera, Lepidoptera, 

 and Diptera. The first group undergoes an incomplete 

 metamorphosis, and the form of the body is fixed from birth ; 

 the second has a complete metamorphosis, and the larvae 

 vary according to the mode of life. They may be agile and 

 slender, plump, 1 and provided with thoracic legs only, 2 or 

 provided with thoracic legs and false abdominal ones 3 ; with- 

 out legs, 4 or without legs and without a differentiated head. 5 

 The different phases of their existence are not always so 

 clearly distinguished as this would indicate. In the aquatic 

 larvae of the Ephemeridae, which carry on the back of each 

 abdominal articulation a pair of scales singularly like rudi- 

 mentary wings, the first signs of wings appear after the first 

 or second, and grow larger with each successive moult. It 

 is the same with the Termites, which are, morphologically, 

 inferior Insects. Hence we may conclude not only that 

 primitive Insects had no sudden metamorphoses, but that the 

 growth of their wings was distributed through the various 

 phases of their life, and that their evolution was continuous 

 like that of other animal organisms. The winged Insects of 

 the present day do not grow any further, but lay their eggs 



1 Campodeiform larvae. 2 Melolonthoid larvae. 



3 Eruciform larvae or Caterpillars. * Helminthoid larvae. 



6 Acephalate larvae. 



