604 ZOOLOGY SECT. 



Distribution in time. The earliest known fossil remains of 

 Insects have been found in rocks of Silurian age. A good many 

 fossil Insects have been found in the Devonian; but they only 

 become abundant in the Carboniferous. All the palaeozoic Insects 

 belong to a group which has been regarded as a distinct order, 

 and has been named the Palceodictyoptera. The members of this 

 group are characterised rather by the absence of the special 

 characteristics of any of the existing orders than by any positive 

 features of their own ; but different families of the order approxi- 

 mate to a certain extent towards the groups of living Insects. 

 Amongst them, for example, are forms representing the Cock- 

 roaches and the Phasmidae among the Orthoptera ; others repre- 

 senting the modern Day-flies among the Neuroptera ; others the 

 Coleoptera. 



Of the existing orders the Neuroptera. Orthoptera, and Coleop- 

 tera are first found in the Trias ; the Hemiptera, Diptera, Hymen- 

 optera, and Lepidoptera in the Jurassic. 



CLASS V. ARACHNIDA. 



The class Arachnida, comprising the Scorpions and Spiders, the 

 Mites and Ticks, the King-crabs, and a number of other families, 

 is a much less homogeneous group than the Insecta, approaching 

 the Crustacea in the variety which it presents in the arrangement 

 of the segments and their appendages. In most members of the 

 class, however, there is an anterior region of the body the cephalo- 

 thorax representing both head and thorax, and a posterior part, 

 or abdomen, which is typically composed of a number of distinct 

 segments ; in some cases cephalothorax and abdomen are amal^ 

 mated. There are no antennas in the adult Arachnid, thou^ 

 rudiments of them have been found in the larvae of some specie 

 The first pair of appendages of the cephalothorax (probably repr( 

 senting the antennae of the Crayfish) are the chelic'erce ; the secon( 

 are the pedipalpi, the representatives of the Crayfish's and Cock- 

 roach's mandibles. Behind these are four pairs of legs. Th( 

 organs of respiration are sometimes tracheae, similar to those oi 

 the Insects, sometimes book-lungs or sacs containing numerous 

 book-leaf-like plates: sometimes leaf-like external appendage 

 or gills. 



1. EXAMPLE OF THE CLASS. THE SCORPION (Euscorpio or 



Buthus). 



Scorpions are inhabitants of warm countries the largest kin< 

 being found in tropical Africa and America. They are nocturnal 

 animals, remaining in holes and crevices during the day, and 

 issuing forth at night to hunt for their prey, which consists of 



