86 THOMPSON YATES LABORATORIES REPORT 
to describe and figure the manner in which the labium was disposed of during the 
puncture of the skin. The stylets probably enter the skin as one piece, being guided 
by the tip of the labium and supported on each side by the basal portions of the 
labellae. The piercing of the skin is brought about by muscular force directed from 
the body of the insect, the muscles attached to the bases of the stylets serving to 
keep them rigid. The withdrawal of the stylets is accomplished by the powerful 
retractor muscles attached to the chitinous prolongations of the maxillae, and the 
muscles described in connection with the bases of the other mouth parts. During 
the process of extraction, while the stylets are slowly sinking into the groove on the 
upper surface of the straightening labium, the insect keeps the labellae pressed firmly 
upon the skin. After they have emerged, the labellae spring together over their tips. 
By a careful study of the minute anatomy of the proboscis, as detailed above, 
it is not difficult to suggest a method by which the mature larvae of F. nocturna may 
escape from the proboscis. As above mentioned the dimensions of this larva are 
roo6 mm. long and 0*025 mm. broad. It is therefore evident, taking into con- 
sideration the dimensions of the several parts of the proboscis, that the most likely 
method of gaining access to the proboscis from the head is by entering the body of 
the labium, the structure and disposition of which would easily admit of this. It has 
been suggested that the larvae lie among the stylets — in which case it will be seen 
from the study of the attachments of these appendages that the larva would in its 
course, necessarily have to pierce a stout layer of chitin, a procedure exceedingly 
improbable. But the evidence that the larvae do reach the labium is now 
conclusive. Low 1 in sections of the proboscis found them there ; and although he 
describes them as ' making an independent passage through the base of the labium 
and pushing forward along the proboscis between the labium and the hypopharynx 
amongst the stylets, where they are found stretched along the length of the 
proboscis head foremost,' the illustrations of his sections of the proboscis shew the 
worm in the body of the labium, and he cannot have been intimately acquainted with 
the minute and most delicate anatomy of these parts. These illustrations certainly 
do not shew the worm ' amongst the stylets,' but in the tissue of the labium. 
Grassi and Noe* often found F. immitis in the labia of mosquitoes (Anopheles 
claviger) which had fed on the blood of an infected dog ; and we ourselves, once in a 
dead mosquito, and again in a living insect, found the larvae alongside the tracheae of 
the labium. 
The question then arises as to how the larva leave the body of the labium 
and reach man, since it must be presumed that their presence in such an organ as 
the proboscis indicates that they subsequently leave that organ during or about the 
time of puncture. Judging from the condition of the larva at this stage, which 
1. Low, British Medical Journal, 1900. Vol. II, June 16. 
2. Grnssi and Noe, British Medical Journal, 1900. Vol. II, p. 1306. 
