SURROUNDINGS OF GROWING INSECTS 203 



their lives, and since insects are pre-eminently aerial creatures, 

 we realize from this study that the aquatic habits of the larvae 

 must be regarded as exceptional, howsoever numerous water- 

 dwelling larvae be. Those larvae like the drone-fly maggot 

 or the gnat grub that breathe by getting contact with the 

 atmosphere are clearly less remote from original conditions 

 than those which like the " blood-worm " or the immature 

 mayfly have the spiracles entirely closed up and are thus 

 dependent on some respiratory organs of branchial type. 



Consideration has already been given to a number of air- 

 breathing insect-larvae of different orders that are adapted for 

 life amid different surroundings by specially appropriate 

 modifications of structure. It may be recalled, for example, 

 how the caterpillars of moths and saw-flies are enabled to crawl 

 along twigs and the edges of leaves by means of their series of 

 paired prolegs affording support for the entire length of the 

 body. Many caterpillars live burrowing in twigs or stems, or 

 mining in the substance of leaves, and where the size of these 

 larvae or the extent of the cavities in which they have to dwell, 

 is such as to avoid any marked pressure on their bodies, there 

 may be no marked modification of their general form. But 

 if a caterpillar of fair size mines in a thin leaf as that of the 

 saw-fly Fenusa pumilio in raspberry canes it may be 

 strongly flattened from above down wards, while its prolegs 

 become short and inconspicuous. The grubs of the wood- 

 boring saw-flies (Siricidae), moving but slowly along their 

 narrow tunnels, have no prolegs, while their thoracic legs are 

 very short and small. Turning to the beetles whose larvae 

 live underground, it is noteworthy how the " wireworm ", 

 with its narrow, elongate body and hard cuticle moves rapidly 

 through the soil, while the heavily-built chafer grubs with their 

 relatively soft cuticle, and the pale, legless weevil grubs lie 

 for long periods curled up in little earthen chambers. The 

 maggot type of larva characteristic of the more highly- 

 organized Diptera is remarkable for the great variety of 

 conditions to which it may be fitted, for we find maggots of 

 essentially similar structure, living in the roots of plants, 

 mining in leaves, burrowing into vegetable refuse on culti- 

 vated land, into the masses of seaweed cast up along the 

 tidal margin, or into animal tissues either alive or dead. The 



