w19 MAUD D. HAVILAND 
forwards and downwards,.and between the first and second 
segments enters a duct lined with epithelial cells, very similar 
to those of the oesophagus (fig. 10). The two ducts unite 
behind the head to form the common salivary duct, which 
opens just inside, on the floor of the mouth. Under high power, 
the ducts have the trachea-like structure found in most insects. 
On either side of the salivary aperture is inserted a small muscle, 
which runs outwards and backwards to the endoskeleton of the 
head. When these contract, the labium, and consequently 
TEXxtT-FIG. 10. 
Longitudinal section through the salivary gland and duct of a larva 
of the fourth instar. x 300. 
the opening of the salivary duct, is slightly everted from the 
mouth (fig. 5). 
Two pairs of buccal muscles are connected with the labrum, 
and by their contraction enlarge the buccal cavity. The 
anterior, and more lateral, pair arise from the exoskeleton of 
the front of the head, just above the labrum, on either side of the 
median line, and running directly downwards (or, having regard 
to the position of the head, backwards) are inserted on the roof 
of the mouth. The posterior and median pair arise together 
behind the last, and, running forwards obliquely between them, 
are inserted on the distal half of the labrum (fig. 18). The 
