SURROUNDINGS OF GROWING INSECTS 215 



gallery ; these tunnels increase in diameter as the larvae grow 

 larger, and terminate in oval chambers wherein they pupate 

 (Plate IV). Each young beetle after its emergence from the 

 pupal cuticle bores its way straight out through the bark, 

 which viewed from outside looks as if pierced by numerous 

 small shot-holes. 



Besides the large number of insect larvae that feed on living 

 plant-tissues, of which a small selection has been mentioned 

 in the preceding pages, very many live on dead or decaying 

 vegetable matter, playing thus the part of scavengers in the 

 economy of nature. In a rotten potato or turnip, for example, 

 there may often be found a crowd of narrow, elongate, whitish 

 grubs with small heads, feeding greedily on the decaying mass. 

 These come from the eggs of Mycetophilidae or fungus-midges, 

 a family of Diptera related to the Cecidomyidae or gall- 

 midges mentioned above. The Diptera are noteworthy 

 among insects for the number of their species whose larvae 

 act as scavengers, and this habit is especially characteristic 

 of the maggot type of larva. The common house-fly (Musca 

 domestica) 1 furnishes the best-known example of this relation- 

 ship. The flies often lay their elongate, white eggs in heaps 

 of decaying vegetable refuse, such as the contents of garden 

 bins, and into this the maggots, when hatched, work their 

 way. A condition of fermentation in the material is especially 

 attractive to the flies about to lay their eggs, and a rise in 

 temperature always hastens the development of the maggots, 

 so that the larval stage may last only a week. House-fly 

 maggots will also feed in dry dustbin refuse, such as papers 

 and rags, or in the remains of old mattresses, but the substance 

 which more than any other serves them for food is horse- 

 dung, which may be defined as animal excrement consisting 

 largely of vegetable remains, because of the grazing and 

 grain-eating habits of the horse. Manure-heaps form, there- 

 fore, highly attractive breeding-places for flies. They also 

 seek exposed human excrement for the purposes of egg- 

 laying, and in this way they readily become carriers of disease- 

 germs into human dwellings and on to human food. The 

 familiar house-fly affords an example of an insect in which 

 the surroundings of the imago often differ strikingly from 



1 C. Gordon Hewitt : " The House-fly ". Cambridge, 1914. 



