28 
0,75 — 0,8 mm. to 0,5—0,6 mm. respectively in adult larvæ). The 
dorsal surface of the air-sacs is strongly convex, the ventral surface 
is slightly concave; the two surfaces meet in two somewhat in- 
wardly curved, slightly conical or rounded ends. The anterior end 
of the anterior sac and the posterior end of the posterior sac are 
always the more inwardly turned, so that the sacs in side view do 
not look symmetrical, they always seem somewhat distorted. This 
inward flexion of the one end of the biadder may increase, pointing 
towards the other end, the concave ventral surface thus becoming 
a rather deep but narrow sloping interspace between the ends of 
the now spiral-shaped air-bladder. (Fig. 1). 
Until the last larval moult, these four air-sacs are almost the 
only representatives of the tracheal system, the rest of the two 
main trunks being only represented by small incoherent cellular 
strings ^), but in the last larval stage they coalesce and become 
tubular and partly filled with air. In the pupation the whole intima 
of both vesicles and trunks is removed. But as the vesicles do 
not seem to be removed with the skin in the preceeding moultings, 
they evidently grow with the animal, as I will try to explain later 
on. As a rule, the form of the bladders of young larvæ seem to 
be kidney-shaped (Fig. 1), the spiral curvature of the one end ap- 
parently increases during growth. Towards the pupation they become 
more siender (Pouchet). 
As remainders of the tracheal system these air-sacs are enve- 
loped in sacs of epithelial tissue which is rather thick at the con¬ 
cave side, thinner at the convex sides. In the dorsal parts and 
frequently at the ends, these epithelial sacs are covered with black, 
or brownish black, generally rounded or irregularly polygonal pig- 
ment-cells, placed in a very thin peritoneal membrane; the nucleus 
h M e i n e r t 1. c. pag. 415; W e i s m a n n 1. c. p. 12. According to Pa 1 m é n 
(1. c. p. 56 -57) the whole tracheal system is present even in the newly 
hatched larva as very tiny cellular strings. In half-grown as well as in 
full-grown larvæ I have seen fine strings which apparently run from the 
anterior to the posterior bladders; but their origin could not clearly be 
seen. They gave off branches, one of which in each segment seemed to 
run to a fine spot in the cuticle. But I find it impossible to decide whe- 
ther they are really rudimentary tracheal tubes, sensitive nerves or strings 
of connective tissue. 
