Fresh-Water Insects 291 



the abdomen depressed, a considerable amount of 

 air is enclosed, allowing the insect to remain sub- 

 merged for some time. Though these beetles spend 

 the greater part of their time in water, they are able 

 to fly, and so make excursions from one pond or 

 stream to another (170). 



Another mode of adaptation to life in the water 

 is shown by the family of bugs known as "water- 

 scorpions " (Nepidae). They are provided with a 

 pair of long-grooved appendages at the tail-end of 

 the body ; these can be closely pressed together 

 and interlocked by means of short hairs so as to form 

 a tube, the tip of which pierces the surface-film when 

 necessary and conveys a supply of air to the spiracles 

 situated at the hinder end of the abdomen. These 

 insects, like the allied "water-boatmen" (Notonectidce), 

 have well-developed wings, and make excursions by 

 night to find a new watery dwelling-place (170). 



Many insects lead an aquatic life only during their 

 larval stage, the imago being adapted for life in 

 the air. Naturally enough, however, such insects are 

 to be found flying chiefly in the neighbourhood of the 

 water whence they came and in which they will lay 

 their eggs — the Mayflies and Midges for example. 

 The contrast between the conditions of the larval 

 and the imaginal life in such cases is most striking, 

 and can only have been brought about by slow 

 degrees. A certain amount of moisture in the earth 

 is necessary to the well-being of many burrowing 

 larvae, while some are found in semi-liquid mud, in 

 decaying refuse, or in animal excrement. In such 

 surroundings breathing through the lateral spiracles 

 becomes impossible, and we find that access to the 

 air-tubes takes place only by one or two pairs of 

 spiracles near the head or tail-end of the body, some- 

 times opening through "respiratory trumpets " whose 



