65 



nervous threads, the whole constituting a nervous chain, from which nerves ramify to 

 the different organs, • enduing them with the various senses they possess ; from the first 

 ganglin the nerves of the eyes and antennae are fed, and from the second the mouth. 

 Hearing and smell are certainly possessed by insects ; but by what organ they are appre- 

 ciated is not ascertained. Taste the seat of which is the mouth, sight in the eyes, and 

 touch probably in the antennae, the palpi, and in the tarsi of the feet. 



The circulatory system is well represented in insects. The blood is generally colour- 

 less, but occasionally of a greenish or reddish hue. The heart, which comprises a series of 

 large reservoirs in the form of a long tube plainly discernible through the transparent 

 skins of caterpillars, lies above the alimentary canal on the upper surface of the body. 



Insects do not breathe, like large animals, through their mouths \ but by means of 

 breathing-holes in their sides, which connect with two great air canals (tracheae) which 

 run along the sides of the body, and from which smaller tubes convey the air in very 

 small volumes to different parts of the body, and so oxygenate the vital fluid in its pas 

 sage. In the Arachnida this tracheal system is considerably modified, respiration being 

 effected among some spiders by ordinary ramified tube-tracheae, and among others in cer- 

 tain sacs or cavities in the abdomen which have been called pulmo-branchiae from an idea 

 that they partook both of the nature of the lungs of the higher animals and the branchiae 

 or gills of fishes ; as, however, the blood does not penetrate these sacs but is merely oxy- 

 genated in its passage by and round them, while it is being re-collected after use and pre- 

 vious to being sent back to the dorsal vessel or heart, these cavities are now believed to be 

 tracheae localized within a peculiarly furnished sac. Tube-tracheae when examined with the 

 microscope are wonderfully beautiful. Traversing as they do the whole bodies of insects, 

 many of which are soft-bodied, they must necessarily be very flexible, and it might be 

 supposed that the rapid movements of the insect might sometimes cause them to collapse, 

 in which case the circulation of the air would be stopped. Upon examination we find 

 that there is a beautiful and simple contrivance which renders this impossible. The tubes 

 consist of a double integument between which runs a hair-like spirally- twisted fibre, just 

 like the coil of wire which is sometimes put inside india-rubber speaking tubes ; this adds 

 considerably to the elasticity of the air vessels, and when these collapse through the move- 

 ments of the insect, it prevents the opposite sides from adhering, and causes them to 

 resume their tubular form as soon as the pressure is removed. There are generally nine 

 pairs of breathing pores or openings through which the air is admitted into the tracheae. 

 These openings are so constructed that it is impossible for dust to enter the tracheae. At 

 the outside orifice is a corneous plate, and inside that is a hollow chamber, and then at 

 the other side of that is another valve. In perfect insects nearly all the air enters through 

 the thoracic spiracles. 



" When an insect is preparing itself for flight, the act of respiration resembles that 

 of birds under similar circumstances. At the moment of elevating its elytra and expand- 

 ing its wings, which are indeed acts of respiration, the anterior pairs of spiracles are 

 opened, and the air rushing into them is extended over the whole body, which by the 

 expansion of the air-bags is enlarged in bulk, and rendered of less specific gravity ; so that 

 when the spiracles are closed at the instant the insect endeavours to make the first stroke 

 with, and raise itself upon its wings, it is enabled to rise in the air, and sustain a long 

 and powerful flight with but little muscular exertion." In the pupal and larval state respir- 

 ation is performed almost equally by all the spiracles ; but in the imago almost all the air 

 enters by those in and near the thorax, so that generally a pinch under the thorax will 

 kill most soft-bodied insects by suffocating them. 



Of the seven sub-orders into which true insects are divided, the one to which the first 

 place is accorded is called Hymenoptera or membrane-winged insects. The members of 

 this sub-order are easily recognized by the structure of the wings which are four in num- 

 ber (see fig. 45), membranous and without either scales or hairs ; the second pair is always 

 smaller than the first and the wings have not so many veins. On their anterior margin 

 they have a row of stiff hooklets by which they are securely fastened to the front wings 

 when in use. The veins in the wings are of great use in determining the genera ; there 

 are, however, some which are wingless ; but may be easily recognized as belonging to this 

 sub-order from their transformations and general structure. 



