INSECTS AND OTHER ORGANISMS 381 



the number of individuals and kinds is very great, as they 

 are in an English woodland during summer and still more 

 in a tropical forest. A clearer view of what happens is 

 afforded by the intensive study of a less thickly populated 

 region, and a full account of the mutual life- relations of a 

 group of arctic creatures including a fair number of insects, 

 is now available in V. S. Summerhayes and C. S. Elton's 

 work (1923) on the ecology of Spitsbergen. We have 

 referred more than once to some of the springtails of that 

 archipelago ; now it will be profitable to see their part and 

 that of other insects in the general life- drama of the district 

 which they inhabit. The scattered plants growing on the 

 stony slopes of these northern islands afford shelter to 

 various springtails and the larvae of small Diptera ; these 

 insects eat the vegetable tissue either in the fresh or decaying 

 state. The mosses and other lowly plants of the bogs and 

 wet tundra supply food for another association of Collembola 

 and grubs of flies ; and so do the lines of Fucus at and above 

 the tide-marks, as previously mentioned (p. 283). Dwarf- 

 willow serves as feeding- ground for a few sawfly cater- 

 pillars, and some parasitic Hymenoptera lay their eggs in 

 the various plant- feeding larvae. On the leaves and blossoms 

 of the Mountain Avens {Dryas octopetala) a single species of 

 Aphid {Scaeva dryadis) Uves by sucking sap, and a hover-fly 

 (Syrphus tarsatus) haunts the neighbourhood, for its maggots 

 feed on the aphids. Erratic ice-borne boulders are of 

 interest in that they " possess self-supporting communities 

 of their own." On them grows a flora of mosses, lichens, 

 and algae among which live small, pale springtails (Folsomia 

 iv.-oculata). These eat the vegetation and are in their 

 turn devoured by two small spiders and two predaceous 

 mites. Many of the insects, as well as the spiders that feed 

 on them, may fall victims to such birds as Sandpipers and 

 Snow Buntings, which themselves serve as prey to the 

 Arctic Fox ; the latter animal is not infrequently hunted 

 and eaten by the Polar Bear, and thus the small insects of 

 these arctic lands form links in the food-chain or *' nitrogen- 

 cycle " which passes from the sparse vegetation to one of 



