STRUCTURE OF INSECTS. 15 



together at certain distances, but which ganglia are often confluent, 

 especially in the imago state: from these knots or ganglia, an infinity 

 of nerves ramify to the various organs, endowing them with the 

 various senses of which insects are possessed. These are, vision, 

 the external organs of which, the eyes, have been already shortly de- 

 scribed ; hearing, which insects clearly appear to possess, but upon the 

 precise organ of which naturalists are not agreed ; smell, of which tlie 

 external organ is also unascertained ; taste, the seat of which resides 

 in the parts of the mouth ; and feeling or touch, whereof it has been 

 generally supposed that the antennae were the chief organs, but of 

 which the tarsi and the palpi have been respectively regarded as the 

 instruments by various eminent physiologists. 



The Digestive Organs consist of an elongated canal, composed of 

 several distinct portions, which have been termed the pharynx, im- 

 mediately connected with the mouth ; the oesophagus, the craw, the 

 gizzard, the stomach, and the intestines, terminating at the anal orifice : 

 there are moreover a number of delicate elongated tubes, or biliary 

 and salivary vessels opening into the digestive canal, the secretions 

 whereof assist in the conversion of the food. The length of the canal 

 varies greatly, being short in the carnivorous species, and often several 

 times longer than the body in the herbivorous ones, whereof an ex- 

 cellent example will be found noticed in the family Coccinellidae. 



The Circulatory System has only recently been proved to exist. It 

 had been long noticed that a series of large reservoirs, easily to be 

 perceived beneath the transparent dorsal skin of many caterpillars, 

 termed the dorsal vessel, and supposed to be analogous to the heart, 

 underwent alternate contraction and dilatation, but it was supposed that 

 no aperture existed from these reservoirs, and consequently that there 

 was no circulation. This, however, at length appears to have been de- 

 cidedly proved, by the researches of Carus, Strauss-Durkheim, Bur- 

 meister, and especially by the powerful microscope of Mr. Bowerbank, 

 whose two papers in the Entomological Magazine sufficiently prove 

 the existence of a circulation of a cold transparent and nearly colour- 

 less fluidj not only in the larvae of ephemera;, &c., but also in the veins 

 of the wings of the perfect Hemerobius. 



The Respiration of insects is effected by means of two great canals 

 (tracheas) running along the sides of the body, beneath the outer sur- 

 face, and communicating with the atmosphere by means of numerous 

 short tubes terminating at or near the sides of the body in breathing 



