CHYLAQUEOUS FLUID OF INVERTEBRATE ANIMALS. 
639 
size those of the blood-proper of the same insect; each cell is thinly charged with 
molecules, but destitute of nucleus. These cells are not accidental productions ; they 
are constant in every specimen. They belong physiologically to the embryonic stage 
of the circulating fluid, as the other (fig. 44) and more complexly-structured cor- 
puscles pertain to the mature blood. They exhibit not the slightest trace of colour. 
Jn another instance of a water-larva they discovered themselves under a kidney- 
shaped figure (fig. 49), bearing no analogy to any variety discoverable in the fluids 
of any species of adult insect. 
The corpuscles of the chylaqueous fluid of the larvae of the Libellulidae constitute 
minute, fusiform, transparent, pellucid bodies (fig. 48), abounding in great compa- 
rative number in the sustaining fluid; they present no nuclear cell, neither do they 
contain granules, two structural characters in which they differ strikingly from those 
of the blood of the perfect insect of the same species. In the instance of another 
water-larva, the fluid under consideration was found thickly charged with small dis- 
coidal bodies, minutely granulated (fig. 51). Many other varieties might be added, 
but enough data have been adduced to sustain the statement that the fluids of the 
larvae of insects are characterized by morphous elements which contrast unquestion- 
ably with those of the blood of adult insects. These, then, are the grounds on which 
it is contended that the embryonic fluids of the Insect constitute a true chylaqueous 
system ; that it is less complex than the true-blood by which it is destined to be suc- 
ceeded ; that its morphous elements are provisional; that its basis and bulk consist 
of water, vitalized by passage through the parietes of the digestive system ; that, 
morphologically, its floating cells bear some relation to the species, but none to those 
of the true-blood by whicb they are to be followed; and that, finally, it is aerated 
before the evolution and independently of the agency of the tracheal system. The 
principal trunks of the tracheae are distinctly visible in the body of the larva long 
before t\\^ stigynnta (by which a communication is established between these tubes and 
the external mediuni) are formed. The air-tubes, however, while yet closed at both 
extremities, become filled with a gaseous substance. How is this curious fact to be 
explained r In the atmospheric larvae they cannot derive their gaseous contents directly 
from the external air, for fluids and solids intervene ; nor in the water-larvae can they 
absorb the air of the surrounding medium, for they are situated too deeply in the 
interior of the body, those few species excepted in which appropriate appendages 
are provided for the exposure of the tracheal system. The inference is thus rendered 
probable that the tracheae of the larvae of Insects, whether their habitat be atmo- 
spheric or aquatic, become first filled with gas from the fluid occupying the visceral 
cavity — not because they already perform the office of aerating that fluid, but because 
their parietes are endowed with the peculiar faculty of absorbing gaseous elements 
from fuids by which they may be surrounded. Distended with gas, in the larva 
stage they subserve the mechanical office of suspending the aquatic species in their 
temporary habitation. The conclusion finally presses upon the mind, then, that the 
