n6 



THE BOOK OF NATURE STUDY 



during the larval life. The wings become more and more con- 

 spicuous with each successive moult, but otherwise the structure 

 remains tolerably uniform during the whole nine or ten months 

 of aquatic life. When pupation is expected, as in the case of 

 the caddis, some means should be provided by means of which 

 the pupa can reach the top of the water. There it remains nearly 

 stationary until the skin, which has become hard and dry, splits 

 and allows the perfect insect to emerge. At first soft and with 

 crumpled, helpless wings, the insect rapidly dries and hardens 

 in the sun, and then, abandoning for ever its empty pupa case, 

 it escapes into the air. 



A few words may now be said as to the life-history of the 

 gnat. An exposed rain-water barrel, or any receptacle left 



FIG. 53. Larva and pupa of the common gnat (Ctilex pipiens). The larva is to 

 the left, head downwards, the natural position, and the pupa to the right, head 

 upwards, showing the two thoracic breathing-tubes. 



exposed in the open and filled with stagnant water, may be 

 expected to yield the egg-rafts of the common gnat. These 

 consist of a great number of little eggs, each like a minute cylinder 

 in form, and fastened together so as to form a floating mass on 

 the surface of the water. The way in which the egg-raft floats, 

 and its resistance to the little ripples which the wind makes, 

 which might be expected to capsize or submerge it, are well 

 worth special study. The larvae, also common in water-butts 



