(152 



ZOOLOGY 



(reckoned as 1) the muscular force of an Insect bears a ratio to 

 its weight of from about 14 to about 23. Insects are also dis- 

 tinguished among the Invertebrata by the keenness of their 

 senses. The sense of sight is, as we should expect from the 

 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 

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



Fig. 531. — Honey-bee (Apis melUfica). a, queen (perfect female) ; b, worker (Imperfect 

 female), and c, drone (male). (After Brehm.) 



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 individii,als, neidcr ivorhers, and soldiers (Figs. 531 



Fii;. 532. — Red Ant (Formica rufa)J male, worker, and female. (After Brehm.) 



and 532), 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 



