THE CIRCULATOEY SYSTEM. 
37 
lating vessel, and prefers to consider it as un vestige 
anatomique.”*' 
Since, however, the more recent labours of Straus 
Durckheim, Carus, and others, we can speak with con- 
fidence on this matter. 
The former showed the muscular character of the 
dorsal vessel of Lucanus cervus, and proved, by the 
presence of valves, that the blood is truly propelled 
forwards. I^ewport also showed the characters of a 
true heart in Sphinw, and Dr. Bowerbank likewise has 
demonstrated the circulation of fluids in the nervures 
of the wings of Ephemera and Chrysopa by means of 
a dorsal heart. f 
The thorax and abdomen of many Aphides are pro- 
vided with certain glandular organs which secrete and 
exude, often in a beautiful spiral form, a peculiar floccu- 
lent substance that has some analogy to silk. 
In some species this substance is so abundant that 
it entirely conceals the insect. In Phyllaphis fagi these 
flocks equal in length the whole body of the insect. 
Occasionally the fibrous character is nearly lost, 
and the material takes the appearance of a fine meal, 
which gives the Aphis a hoary or pulverulent aspect. 
Two or three species follow the habits of some Lepi- 
doptera in stripping their bodies of this silky coat, 
for the purpose of covering their eggs. Such an 
example may be seen in Chermes laricis. Some Cocci 
also very beautifully conceal themselves by a similar 
device. 
* Dufour remarks, 1. O’est un fait etabli, je crois, en principe, que 
dans les animaux on il y a une circulation generale d’air, celle-ci 
remplace ou exclut la circulation generale du sang on d’un liquide 
analogue. Ces deux systemes circulatoires simultanes sont incom- 
patibles. Je m’abstiens d’examiner les consequences qui decoulent 
naturellement de cette proposition ; elles sont par trop evidentes. 
2. J’ai deja avance que dans Tecbelle entomologique les Hemipteres, 
comme insectes suceurs, avaient une organisation moins compliquee, 
moins parfaite, que celle des insectes broyeurs. La consideration du 
cordon dorsal vient a Fappui de mon assertion, car il est encore plus 
simple, plus grele, plus rudimentaire, cbez eux que dans les insectes a 
mandibules. Vide Leon Dufour, p. 276, “ Recbercbes Anatomiques,” op. 
cit. 
t Vide Bowerbank, ‘ Entom. Mag.,^vol. i, p. 244, and vol. iv, p. 179. 
