190 THE BIOLOGY OF INSECTS 



while their maxillae retain the typical form with slight 

 modification, the lacinia or blade, absent in Lepidoptera 

 generally, being well developed. 



The contrasts in feeding habit between insects of the 

 same kind during the immature and adult periods of their 

 lives suggests the mention of the still greater contrast 

 afforded by many insects which do not feed at all after com- 

 pleting their transformations and acquiring their wings. 

 In most insect life-histories the preparatory stages extend 

 over a far longer time than the duration of adult life. Dragon- 

 fly larvae often spend several years under water before 

 emerging into the air in readiness for the winged insect's 

 flight of a few weeks or months, while the underground 

 life of the cicad already mentioned in this chapter is pro- 

 longed for thirteen or seventeen years, the winged adult 

 dying before the winter of the year in which it comes up. 

 In most metamorphic insects with a yearly life-cycle the 

 life of the imago is much shorter than that of the larva, the 

 former to be reckoned usually in weeks and the latter in 

 months. We have seen that the larval period of the life- 

 history aflFords an opportunity for eating and digesting food 

 and storing it up in the tissues, so that there may be ample 

 supply for the extensive re-making of the creature at the 

 pupal period. Apart from the exceptional precocious modes 

 of reproduction to be considered later in this chapter, 

 pairing and egg-laying are unknown until the insect has 

 reached its adult condition, so that the imago may be 

 regarded as essentially performing the function of repro- 

 duction. It is not surprising, therefore, from this point 

 of view to find that in a number of insects — ^whether exo- 

 pterygote as the Mayflies, or endopterygote as the Silkworm 

 Moths (Bombycidae and Saturniidae) and the botflies 

 (Oestridae) — the imago when developed has the jaws so 

 excessively reduced that it is incapable of taking food, and 

 its activity is entirely concerned with breeding, the feeding 

 necessary for the accomplishment of its life-purpose having 

 already been performed during the larval stages. It has 

 often been remarked that the power of flight, acquired by 



