xi PHYLUM ARTHROPODA 637 



elaborate character of the optic organs, the most highly developed, 

 many Insects having been shown by experiment to have a keen 

 sense of colour ; but a sense of smell, the seat of which is in the 

 antennae and palpi, can be shown to exist in a high degree, and 

 the parts about the mouth bear nerve-endings concerned in a well- 

 developed sense of taste. A sense of hearing does not appear to 

 be universally present, but is well marked in such forms as produce 

 sounds. At the same time Insects are remarkable for the instincts, 

 often leading to results of an elaborate character, which guide 

 them in the pursuit of food and the protection and rearing of 

 their young. Among the insects which are the most highly 

 endowed in this respect are some the Ants, Bees, Wasps, and 

 Termites which live together in organised associations or com- 

 munities, the various individuals composing which are distinguish- 

 able into sexual individuals, neuter workers, and soldiers (Figs. 541 

 and 542), each specially organised for the part which it has to play 

 in the economy of the community. 



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 

 Coleoptera are first found in the Trias ; the Hemiptera, Diptera, 

 Hvmenoptera. 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 amalga- 

 mated. There are no antennae in the adult Arachnid, though 



