AEACHNOIDEA. 101 



the very similar lulus Londinensis (fig. 38, b), whioh has a round 

 tail. Both feed upon the rootage of various plants. 



The genus Polydesmus forms a link between the two groups 

 of Myriapods. The species are flat and compressed, instead of 

 being rounded as in lulus. The young develop as in lulus, but 

 the legs are on the second, fourth, and fifth segments in the 

 young specimens. Polydesmus complanatus, a pale purplish- 

 white or rosy-tinted species, nearly an inch in length when 

 mature, has similar habits to lulus, feeding especially upon 

 onions, and often causing their decay. I have received notice 

 of much damage caused by them in greenhouses. 



The Chilopoda or Centipedes are all beneficial. The family 

 Seolopendridce is represented by four genera in England, all 

 being more or less abundant. One of the commonest British 

 species is Geojjldlus longicornis of Leach, which is a long, 

 yellowish, thread-like creature, often two inches in length. It 

 moves rapidly, with a curious, sinuous, snake-like movement. 

 The eggs are laid in a little cell in the earth, and are looked 

 after by the female, who is said never to leave them for the two 

 weeks they are incubating, coiling herself round them in the 

 cell. This species may often be turned up in the earth whilst 

 digging in a garden, when by its great activity it soon will bury 

 itself again. Their food is composed of other ground-insects, 

 snails, slugs, and small worms. Some species are luminous. 



Arachnoidea {^Spiders, Scorpions, and Mites). 



The three chief groups of the Arachnoidea are the Spiders, 

 Scorpions, and Mites. The first and last only are of any econ- 

 omic importance. Another small division known as the Penta- 

 stomidcB must also claim a few passing words, as they sometimes 

 cause ill effects and loss in farm stock, living as parasites in 

 the sinus of the bones and air-sacs of animals and birds. The 

 distinguishing feature of the Arachnoidea is the presence of four 

 pairs of legs, which are nearly always found in the adult. There 



