296 Insects and their Surroundings 



fringed with long hairs. The elongate wingless 

 fore-body of these insects and the greatly reduced 

 hind-body give them a most peculiar and character- 

 istic appearance, and the dense pile wherewith they are 

 clothed keeps them dry. They have been observed 

 gliding over the calm seas of the tropics, often 

 hundreds of miles from land, or clinging to drifting 

 substances whence they could suck food. Their eggs 

 have been picked up attached to the floating feather 

 of a sea-bird. 



Geographical Distribution. — Having seen in 

 what diverse situations insects are to be found we 

 turn next to consider their distribution over the earth's 

 surface. The range of any insect is, of course, 

 limited to some extent by its habits and its food 

 supply. Mayflies cannot be expected to occur in the 

 midst of a sandy waste, nor wood-boring Beetles in 

 a region where there are no trees. But apart alto- 

 gether from such limitations as these the distribution 

 of species, genera, and families of insects, has much 

 to teach the student. Some groups are almost world- 

 wide in their range, others discontinuous, others 

 confined to a single district, though there is nothing 

 in their habits or feeding to call for such restriction. 

 Within our own islands we can find instances of these 

 various types of distribution. A vast number of 

 species are found everywhere that the conditions 

 render their presence possible. Some — like the great 

 Water-beetle Hydrophilus piceus and the Purple Emperor 

 Butterfly Apatura iris — are never, or very rarely, to be 

 seen northward of the Trent or westward of the 

 Severn. Others, on the contrary, such as the burnet 

 moth Zygatia 7ni?ios and the boring-beetle Alesites 

 Tardyi, are entirely absent from the south-eastern 

 districts of Great Britain while they abound in certain 

 western and highland regions and in Ireland. Com- 



