334 THE BIOLOGY OF INSECTS 



insects, and as the sub-imago of a mayfly is the penultimate 

 instar it corresponds with the pupa in the great meta- 

 morphic orders, though its comparatively active habits offer 

 a strong contrast to the mainly passive behaviour of the 

 latter. 



The pre-pupal and pupal periods in the life-histoiy of 

 those insects that undergo a '' complete " transformation 

 (metamorphic or ** holometabolous " insects) allow, as we 

 have seen (pp. 173-6), opportunity for the extensive 

 reconstruction necessary for the development of the winged 

 imago from a larva that diverges wddely from it in details 

 of form and manner of life. The degree of divergence 

 between larva and imago may be taken generally as a measure 

 of the degree of specialisation attained by the various orders 

 of the Endopterygota, and this divergence is often clearly 

 very great in those orders whose members show the highest 

 specialisation of structure in the adult. For example, the 

 Diptera with their elaborate sucking jaws, their concentrated 

 body-form, and the remarkable reduction of their hind- 

 wings, the forewings alone being used in flight, are de- 

 veloped from legless grubs or headless maggots, in many 

 respects degraded yet often remarkably adapted for special 

 methods of feeding. The higher Hymenoptera, which also 

 show great modification in their body-structure, wings, 

 and suctorial labium, offer in their adult condition a striking 

 contrast to the small-headed, soft-coated larvae which seem 

 so sluggish and inactive because the nursing activities of 

 their mother or elder sisters have provided for them an 

 abundant and appropriate food supply. The active, 

 armoured *' thysanuriform " hrvsL, characteristic of many 

 families of beetles and Neuroptera, is clearly much closer 

 than the fly-maggot or the bee-grub to the primitive insectan 

 type. All the thysanuriform larvae among the Endopterygota, 

 however, are without abdominal appendages ; these may be 

 represented in a greatly modified form by the pro-legs of 

 caterpillars, especially those of the scorpion-flies (Mecoptera, 

 Fig. 78) and of the most primitive of the Lepidoptera 

 (Micropterygidae) in whose larvae the pro-legs are im- 



