Gr . H. F. Nuttall 
809 
Mechanism. 
In considering the chitinous structures we have already had to touch 
upon matters relating to the mechanism but there are other points still 
to consider, especially the mechanism whereby the apparatus is protruded 
and retracted. 
The mechanism whereby 'protrusion is ejfected. When the living male 
is handled, as Leeuwenhoek and apparently no others since have ob¬ 
served, the point of the dilator is protruded to a moderate degree in 
the course of muscular efforts made by the insect to free itself. This 
point is the first part of the apparatus to be protruded, and males 
approaching females may be seen to protrude it intermittently in a 
similar manner. This movement is effected by a system of parallel 
muscular fibres, of great length, which arise at the posterior (ventral) 
part of the abdomen where it curves dorsally, and run forward to their 
insertions upon the ventral surface of the anterior or rounded end of 
the basal plate (Text-figs. 2, 3, M. protr. pi.). When a male is viewed 
ventrally, or the parts are dissected out, these protractor muscles are 
seen to cover the whole ventral surface of the plate, the fibres running 
parallel to each other, a few fibres running alongside the edge of the 
plate. When viewed in longitudinal sections the muscles are seen to 
spread cut in a fan-like manner as they approach their points of origin 
on the wall of the abdomen. This wall, beneath the basal plate, is 
traversed by numerous creases which permit of considerable extension 
of the surface and only the deeper creases remain clearly in evidence 
when the copulatory apparatus is extruded (Text-figs. 1, 2, 3, tr.f.). 
As the dilator is further extruded it becomes flexed upon the basal 
plate. This movement is no doubt in the main due to the traction 
exerted through the collar-like membrane connecting the dilator with 
underlying structures, mainly a system of powerful flexor muscles (Text- 
fig. 2, M. flex, dil.) which run obhquely backward and upward from the 
ventral abdominal wall to their insertion in the collar-membrane which 
serves as a tendon. These muscles do not, however, come into full play 
until a later stage, for the male starts the process of copulation by inserting 
the dilator only and it is not until the vesica is fully everted that he 
depresses the dilator fully by the action of these muscles so that the tip 
of the dilator rests upon his back. These flexor muscles of the dilator 
are only shown schematically, as in Text-fig. 2, as three fibres, whereas 
in Text-fig. 4, drawn from an actual dissection, they are shown torn 
away from their abdominal attachments and much contracted; the 
