INTEODUOTION. 15 



by the contractions of the heart, and carried to the head by the 

 aortic portion of the dorsal vessel, can only distribute itself in the 

 different parts of the system to return to the heart, by the gaps 

 left between the different organs, or the membranes and fibres 

 of which these organs are composed. 



Fig. 13 (page 17), which shows both the circulating and 

 breathing systems of an insect, enables us to recognise the 

 different organs which we have described, as helping to keep up 

 both respiration and circulation. 



The knowledge of the respiration of the insect is a scientific 

 acquisition which is quite modern. Malpighi was the first to 

 prove, in 1669, that these animals are provided with organs 

 of respiration, and that air is as indispensable to insects as it is to 

 other living beings. But the opinion of this celebrated naturalist 

 has been contradicted, and his views have been contested, even in 

 the present day. Now, however, one can easily recognise the 

 apparatus by the aid of which the respiration of the insect is 

 effected. 



In all these animals the respiratory apparatus is essentially 

 composed of membraneous ducts of great tenuity, of which the 

 ramifications in incalculable numbers spread everywhere, and 

 bury themselves in the different organs, much in the same way as 

 the fibrous roots of plants bury themselves in the soil. These 

 vessels are called tracheae. Their communications with the air 

 are externally established in different ways, according to the cha- 

 racter of the medium in which the insect lives. 



It is well known that the greater part of all insects live in the 

 air. This air penetrates into the tracheae by a number of orifices 

 placed at the sides of the body, which are termed spiracles. On 

 close examination these may be seen, and are in the shape of 

 button-holes in a number of different species. Let us dwell for a 

 moment on the breathing apparatus of the insect, that is to say, 

 the tracheae. 



This apparatus is sometimes composed of elastic tubes only, 

 sometimes of a collection of tubes and membraneous pouches. 

 We will first speak of the former. 



The coats of these breathing tubes are very elastic, and always 

 preserve a cylindrical form, even when not distended. This state of 



