OF CHLOËON (EPHEMERA) DIMIDIATUM. 481 
32 
“The pulsation,” he says, “of the dorsal vessel cannot be essentially affected 
by these movements, as, when they cease for a longer or shorter time, the activity 
of the dorsal vessel does not stop. This applies also to the action of the digestive organs ; 
for this likewise goes on, and movements (especially peristaltic) of the intestine and 
malpighian vessels take place, even when the movements of the abdomen cease for some 
time. There can also be no particular relation between them and the function of the 
generative organs, as they occur both when the sexual organs are far from maturity 
and when their activity appears to be purely plastic. There are no other organs in the 
abdomen of most insects, except the respiratory organs, with which these movements 
can be connected ;" and he concludes, therefore, that the function of their movements is 
to produce a change of air in the tracheæ, and that, “ from the absence of all such phe- 
nomena we might conclude that in the pupæ of the above-mentioned insects (Beetles 
and Hymenoptera) the tracheary respiration is entirely interrupted ” *. 
But in the paper above mentioned, I have shown that the mode in which the tracheæ 
branch offers no support to the idea of any circulation of air, while the fact that they 
end in long blind extremities seems to be fatal to any such hypothesis. Under these 
eireumstances, it occurred to me, as indeed had been suggested long before by Dr. 
Graham, that the interchange of air was effected by virtue of the diffusion of gases. I 
see no reason to change this opinion, which, moreover, receives very strong support 
from the arrangement of the respiratory organs of the Chloéon-larva and other aquatic 
larvg which respire by means of gills. Here there are no external openings, conse- 
quently there can be no inspiration or expiration of air; and yet it is evident that the 
respiration is not interrupted. 
I will not deny that in aérial insects a partial change of the air in the larger tracheæ may 
be effected by the “ respiratory " movements, nor will I assert that there are * any other 
organs in the abdomen” ...... with which these movements can be connected. 
But it seems very possible that they may tend, and be intended, to assist the weak 
“dorsal vessel” in producing the circulation of the blood. ; 
In considering the gills, it is remarkable how small a portion of the surface is occupied 
by the tracheæ. Plate LVIII. fig. 11 represents a portion of the anterior gills ; and it will 
at once be seen how large a part of the surface is apparently wasted. This isa fair eX- 
ample, and quite as richly supplied with air-vessels as is usual. It was copied imme- 
diately after the death of the insect, and before any material change had taken place. kg 
Younger larvæ it might be said that the organ was partially embryonic, and E : 
large extent of membrane was in preparation for a richer network of tracheæ. pri 
ever, we have now reached the definite condition of these gills, which wil be et n he 
at the next change of skin, this argument does not apply. Nor is this an excep re 
“se; the same poverty, if I may use the expression, is visible in en "i 
Organs of other water-insects, and may perhaps be taken as collateral Hope gg 
© modus operandi of these organs is not yet perfectly understood—and eg 
We bear in mind the richness in blood-vessels of the lungs of higher a 
* Ann. & Mag. Nat. Hist. ser. 3. vol. ix. pp- 95, 105. 
