CHAP. VIII. MIGRATION OF LAND CRABS. 267 



has been known to cast a slight shade over a field of four 

 acres, as they passed ; and a host of the common froth 

 cicada (C. spumaria}, which flew in the night, was at 

 first mistaken by professor Walch for a shower of hail 

 pelting against his study window : this continued for 

 at least half an hour. The same writer witnessed, in 

 August, a similar emigration of myriads of a predacious 

 beetle (Carabus vulgaris Lin.). The Aphides, or plant 

 lice, are another migratory family. White of Selborne 

 mentions a flock which passed his village on the 1st of 

 August, 1785, so numerous as to cover such persons 

 as were in the open air at the time, and blackening 

 all the surrounding vegetation. The ladybird beetle 

 (Coccinella septempunctata Lin.) is the deadly enemy 

 of the plant lice, which they appear to follow, on 

 these occasions, in immense armies. These, and many 

 other instances of insect migration, have been given 

 more in detail by Messrs. Kirby and Spence, to whose 

 entertaining pages we may refer the curious reader. 

 The land crabs of the West Indies afford a remarkable 

 instance of migration among apterous insects. They 

 are generally found, in great numbers, in holes and 

 cavities among the mountains ; but every spring they 

 descend, in immense bodies, to the sea coast pursu- 

 ing so direct a line to the place of their destination, 

 that scarcely anything will divert their course ; even the 

 most formidable obstacles are overcome by their per- 

 severance. They march, according to a popular writer, 

 generally in three divisions, with the utmost regularity, 

 and under the guidance of one chosen commander ; 

 the first being composed of the strongest males ; the se- 

 cond consisting of females, which are sometimes formed 

 into columns fifty or sixty yards broad, and three miles 

 deep*; and the third being made up of a mixed, un- 

 disciplined, and less vigorous tribe, which follow a few 

 days afterwards. When they have effected the purpose 



* Upon what authority the ingenious compiler of the Animal Biography 

 gives this statement, we know not. We suspect it may be a little em- 

 bellished ; but not having been in the West Indies, we cannot say any. 

 thing of these land crabs from personal observation. 



