PHYLUM ARTHROPODA 



603 



1;. 474), which inhabit the alimentary canal of mammalian hosts 

 rses, Oxen, Sheep, Rhinoceroses, Tapirs), 

 i accordance with the high grade of the structure of their 

 various systems of organs, Insects exhibit a correspondingly high 

 -degree of functional activity. The quantity of food consumed 

 assimilated is great in comparison with the bulk of the body, 

 the energy expended in muscular contractions is of very con- 

 erable amount. It is estimated that while the muscular force 



sid( 



!>;. 4U3. Honey-bee (Apis mellifica). a, queen (perfect female); b, worker (imperfect 

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



exerted by a Horse bears a ratio of about 0*7 to its own weight 

 {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 

 antennas, 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 



FJ.;. 494. Red ant (Formica ruf a) ; male, worker, and female. 



of food and the protection and rearing of their young. Among 

 the insects which are the most highly endowed in this respect are 

 home the Ants, Bees, Wasps, and Termites which live together 

 in organised associations or communities, the various individuals 

 composing which are distinguishable into sexual individuals, neuter 

 workers, and soldiers (Figs. 493 and 494), each specially or- 

 ganised for the part which it has to play in the economy of the 

 community. 





