36 



with the labium, somewhat anterior to the point where it joins the epipharynx 

 above, to form the mouth. 



Mandibles and maxillae, as free mouth-parts, are absent in Musca vomitoria. 

 The one-jointed maxillary palpi (fig. 1, mp), two club-shaped appendages clothed 

 with a few hairs, are attached to a slight thickening, or band, of the flexible 

 membrane which bridges the upper anterior portion of the fulcrum. This band 

 is marked by longer hairs than the rest of the membrane. The attachment of 

 the maxillary palpi is shown on fig. 1, /?'. 



The labium of Musca vomitoria (side view in fig. 1 ; cross-section in fig. 1, 

 ', /) is a deep trough or channel, the distal end of which is closed entirely over 

 so as to form a tube; to the lateral margins of the distal end of this tube are 

 attached the labellae. The walls of this trough (fig. 1, a', I) are double, that is, 

 the trough itself can be compared to a cylinder of which the upper side has been 

 so invaginated as to form a channel. These double walls extend into the tubular 

 distal portion of the labium, so that the latter portion, structurally considered, is 

 made up of one tube inside another, the greater part of the space between the 

 tubes being beneath the inner one. The labellae are two lobiform hollow sacks; 

 their inner margins are continuous with the inner tube above-mentioned, and 

 their outer wall with the outer tube, so that their inner cavity, is, on each side, 

 a continuation of the space between the two. tubes. The basal channeled end of 

 the labium is attached to the under side of the base of the hypopharynx above, 

 and is continuous below with the lower half of the membranous sheath around 

 the fulcrum. Through the labium extend stout muscles and two tracheal stems 

 (fig. 1, a', /). From the lateral edge of the inner chitinous tube, or wall, of 

 the distal end of the labium there extends into the cavity of each labella a 

 delicate curved chitinous band; on the bases of each of these bands are inserted 

 muscles, which originate in the labium further toward its base. These muscles, 

 altho not extending into the labellae, help to separate the latter from each other. 

 The labium and labellae are somewhat hairy on the outer surface; especially is 

 this true of the margin of the labellae. Each labella bears, on its inner surface, 

 about thirty pseudotracheae, arranged nearly parallel to one another from the 

 outer to the inner margin of the labella, where they, in part, join anteriorly a 

 large pseudotracheal stem, extending toward the middle of the disk formed by 

 the labellae, and, in part open directly into the triangular central opening between 

 the labellae, their basal ends being supported by rods which radiate from a 

 strongly chitinized ring around this opening. Between the bases of the rods 

 supporting the inner ends of the pseudotracheae are teeth, which are concealed 

 when the labellae are not opened very widely. 



