360 THE TRACHEAL SYSTEM OF THE IMAGO. 
the tubes. The tubes and sete are the terminations of the 
ducts of a series of wax glands, situated around the spiracle. 
The glands are convoluted tubules, and contain cells similar to 
those of the sebaceous glands of vertebrates (Pl. XXII., Fig. 8). 
The wax is poured out on the surfaces of the valve-plates and 
over the adjacent integument ; it serves to seal the valve, and 
prevents the adhesion of water to the spiracle and the sur- 
rounding integument. If a living fly is immersed in water a 
large bubble of air always adheres to each of the spiracles. 
After immersion in alcohol this no longer occurs. 
I use the term wax rather than oil, but have no evidence of 
the real character of the secretion, except that it is fatty, is 
intensely blackened by osmic acid, and is soluble in alcohol 
and ether. 
The internal valve consists of a calliper-like sclerite, enclosed 
Description OF PLATE XXII. 
Fic. 1.—The posterior thoracic spiracle of the Blow-fly seen from within the thorax : 
m, muscle which closes the internal valve; 7! the elevator, and m® the de- 
pressor muscle of the halter ; 0, orifice between the open external valve plates ; 
S, 5, Spiracular sac, 
Fic. 2.—The external valve of the posterior spiracle seen from its outer surface. 
Fic. 3.—A portion of the external valve plate showing the hollow branching ribs and 
setee in which the wax ducts terminate, seen with a } objective. 
Fic. 4.—The anterior thoracic spiracle seen from within the thorax. The spiracular 
sac has been removed from the right side to show the external valve plate and 
the calliper-like rod which supports the internal edge of the sac and acts as a 
valve. The small muscle which controls the internal valve is seen below. 
Fic. 5.—The spiracular sac and internal valve of the anterior thoracic spiracle, with 
the origin of the tracheal trunks from the anterior spiracular sac: s, the sac ; 
the muscles have been removed from its upper half to show the wax glands which 
lie at the outer margin of the spiracular sac ; ¢7, tracheal trunks. 
Fics. 6 and 7.—Sections through the anterior spiracles of the nymph: ¢, ¢, the pupa 
sheath ; 2, ¢, intima of the trachea of the nymph; , f, peritoneal coat which 
forms the tracheal intima of the imago; s,s, spiracular sac of the imago ; 
s.¢, stigmatic cornu of the pronymph; sf, spiracular cornu of the nymph ; 
Z, ¢, lumen of trachea of the nymph. 
Fic. 6.—A section through the anterior spiracle of the nymph on the twelfth day of 
the pupa. 
Fic. 7.—A similar section through the anterior spiracles of the nymph, about the 
seventh day of the pupa stage. 
Fic. 8.—A portion of one of the wax glands of the imago seen with a ,'y inch oil 
immersion objective. 
All the figures are drawn with an inch object glass except when otherwise 
stated. 
