1880. | The Proboscis of the House-fly. 159 
Thus we find that the protrusion and distension of this impor- 
tant organ is a joint affair, the tracheal system and the muscles 
combining their services. 
Homologies—It is strange that no previous observer seems to 
have been struck by the evidence of the “ great tendons” of the 
operculum as to what is the organic base of the mouth parts. 
These tendons are found, so far as we have observed, throughout 
the Diptera; they are evidently the tendons of some of the mouth 
parts, marking their origin. They could not be in the mid joint 
of an appendage; the muscles which move the segments of any 
arthropod appendage on each other, are zz¢ernal; it is only when 
we get to their root that we find these tendons extending into the 
body of the animal. Hence we conclude that the mid segment 
is the true base of the fly’s proboscis. We may, perhaps, go fur- 
ther and hold that these great tendons belong to the mandibles, 
for they closely resemble the mandibular tendons of other orders 
of insects and of the lobster. This will make the operculum rep- 
resent two united mandibles, probably enclosing the labrum.’ 
The palps seem to point out the maxillz, but it is not easy to 
determine to which of the hard pieces they belong, as they are 
borne on slight indurations of the membrane. The axis piece, 
With its trabeculoid bars, seems to represent the maxillze with its 
inner and outer processes. The hypopharynx and mentum offer 
no difficulty. The small piece represented in Fig. 3 in advance of 
the fulcrum, may belong to the maxilla. The membrane sheath 
and tip with spreading lips may be regarded as the labium with 
its specially developed paraglossz. 
Having made out the chief mouth parts as represented by the 
: 1 Gegenbaur approaches this discovery in commenting on the tracheal system of 
insects in water. The branchial tracheæ are kept distended in water by inflation, _ 
and he thinks that the tracheal system has a “ hydrostatic function,” which, in some — 
cases, may be more important than their respiratcry function. It is probable that the 
trachez of insects serve more purposes than we have yet recognized. Tn 
See Prof. Fackard’s account of the hydrostatic functions of the larval tracheæ of 
Corydalis, which illustrates and confirms Gegenbaur’s view. AMERICAN NATURAL- 
IST, Vol. vin, p. 53 a e. y 
* Huxley Says (Anatomy of Invertebrated Animals, p. 427; in American edition, 
P- 369): “In the common house-fly the labrum, mandibles and maxillæ coalesce at 
their origins and constitute the base of the proboscis, which is mainly formed by the 
confluent second maxillx, Its longitudinal grooved anterior face is overhung by the — 
elongated styliform labrum.” It is probable that the author was misled by endeavor- 
ing condense the views of others in this part. Anything done by himself would os : 
Scarcely be so faulty as this passage unquestionably is. se ple ee 
