36 
two races of larvæ differing as well in morphological as in physio- 
logical and biological respect. 
It is an interesting fact that while the air-sacs of the land insects 
have very thin walls without any spiral ribs, the air-sacs of the 
aquatic larvæ -— at least those of the Culicidæ — have particu- 
larly thickened walls with highly developed spiral ribs. The sacs 
of land insects must be distensible and capable of holding a good 
deal of air, when the insect fills its tracheal system, no matter if 
they collapse when emptied again; whereas the sacs of the aquatic 
larvæ must be able to stand an outer pressure of the water with- 
out being compressed. 
There is still one problem left about the air-sacs of Corethra. 
Being only connected with the cuticle by very tiny cellular strings 
(and in the young larva perhaps with no connection at all), through 
which — even if they become tubular towards the moulting — it 
seems quite impossible for the stout and strong air-sacs to be re- 
moved at the moulting, they evidently must increase within the 
larva during its growth”); the bladders of a full grown larva are 
several times larger than that of a newly hatched one, as it also 
might be expected. But how do they increase? The chitine being 
a dead product of the epithelial cells, can grow no more, when 
once it is formed. — No doubt there are four possibilities for the 
increasing of the chitine wall of the bladder. 
1% As in the Vertebrata the osteoclasts are constantly eating 
some parts of the bones away during the growth, the chitine in the 
air-bladders might be removed by some destructive cell elements, 
while the epithelial cells constantly formed new layers of chitine 
of increasing size. But evidences of a destruction like this are 
never found,. either in the epithelial cells or in the chitinous intima. 
2%.  While' the chitine wall is being constantly formed by the 
activity of the epithelial cells its free surface might constantly moul- 
der away, the wall thus keeping always nearly the same only slightly 
Y According to Palmén (Il. c. pag. 57) the intima (of the bladders?) break 
seen several larval moults, he has never found the sacs re- 
moved with the exuvia. Also my own experiences lead to the same 
supposition that they are not removed until the pupa change. 
