PSYCHE. 
of pneumatic efflation. The pressure of 
air from within gives definite form to the 
body ; puffs up the forehead into soft blad- 
der-like bumps, and swells out the wings 
like soap-bubbles, until the two sides come 
together and harden into the fine wing- 
membrane with its double plate and en- 
closed tracheae. This shows the insect’s 
wing to be merely an outgrowth (exodeme) 
of its body-wall like the pleura of a lob- 
ster. 
The scale-like larva of the beetle Pseph- 
enus (once described as 
Fluvicola) which abounds on the loose 
rocks in our streams, has brush-like tra- 
cheal filaments (external to its abdomen) 
which sweep the water of its oxygen, and 
may be observed swelling out and ‘‘kick- 
ing” at every pulsation; a result of pneu- 
matic pressure within the body. 
Réaumur observed that young flies can 
at will inflate a sac on their foreheads, 
expanding and contracting it. 
a crustacean, 
Weismann, 
in his study of the embryology of diptera, 
made an observation which seem to me to 
belong to the same category. The jaws 
of diptera arise far back in the body, pro- 
jecting like limbs, before the true limbs 
show themselves. Subsequently the jaws 
travel headwards, become directed for- 
wards instead of transversely, and sink 
out of sight within the oral cavity which 
is now formed by invagination. Then the 
head itself is swallowed up in the trunk, 
being lost to view, and thus we have the 
well-known headless maggot of these in- 
sects. In the subsequent growth of the 
head-parts this author thinks that there is 
a complete discontinuity between the larva 
8 Zeitschr. f. wiss. Zool., 1863, 1864 and 1866; 
bd. 13, 14 and 16. 
‘developed towards the head. 
377 
and the adult, a histolysis or disintegration 
of the tissues, which must make a new de- 
parture in order to continue their growth. 
Balfour suggests that, instead of a complete 
break, we may have here only a skipping 
of intermediate stages. ‘The rule of em- 
bryo-life is that when a part has a long 
journey before it to reach high organ- 
ization, it starts early, takes all short cuts, 
and does not delay at the ‘‘ way-stations.” 
My own observations point to the conclu- 
sion that the fine chitinous frame-work and 
ledges of the introverted head of the larval 
fly fairly forecast the characteristic struc- 
tures of the adult, and that the plates of 
the basi-proboscis occupy in the larva the 
normal place of an endocranium (with 
which I deem them homologous).°- Weis- 
mann states that he was unable to trace 
the development of these parts, because 
the head was invisible ; and hence he could 
not work out their homologies. 
He found, however, that the head came 
forth to view again from its introverted 
position by the influence of mechanical 
pressure. The abdomen contracts, and 
this drives the contents of the body for- 
wards so as toeject the head. He specifies 
only the fluid contents ; but whatever oper- 
ates onthem must a fortiori set in motion the 
pneumatic dynamics of the tracheae which 
are then distended with air and largely 
Hence the 
whole head is, in the first instance, driven 
out by the same aerial impulse which 
subsequently inflates the frontal sac and 
propels the proboscis. 
The same authority states that the fine 
branches of tracheae in the systern termi- 
nate in spindle-cells with thin elastic 
9 Amer. naturalist, March 1880, v. 14, p. 160. 
