108 LETTERS ON ENTOMOLOGY. 
wings, poisers and sometimes winglets. The 
poisers are little membranaceous threads, placed 
one under the origin of each wing, near a spiracle 
or breathing hole, and terminated by an oval, 
round, or triangular button, which seems capable 
of dilatation and contraction. The winglets are 
of rigid membrane, and fringed, and generally 
consist of two concavo-convex pieces situated 
between the wing and the poisers, which, when 
the insect reposes, fold over each other like the 
valves of a bivalve shell ; but when it flies they 
are extended. If either of the winglets or poisers 
are cut off, the insect flies unsteadily, and leans 
to one side. The buzzing or humming of a fly 
is produced by the vibration of the root of the 
wings, and in most instances by the winglets 
and poisers. Though in their general appear- 
ance flies greatly resemble each other, yet in the 
form of the head and organs of feeding there is 
a material difference. Some have a trunk with- 
out any teeth ; some have a mouth without either 
trunk or teeth ; others have a mouth furnished 
with teeth, and others have both trunk and 
teeth. To give you a few examples. All the 
bee species have a trunk and two teeth above it : 
all the tribe of wasps have a mouth and two 
teeth inside. The plant-lice or aphides, which. 
