INTRODUCTION. 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 toe 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 con- 
tested, 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 membranous 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 trachee. Their communications with the air 
are externally established in different ways, according to the 
character 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 trachez 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 trachez. 
This apparatus is sometimes composed of elastic tubes only, 
sometimes of a collection of tubes and membranous 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 
