APTERA. 437 
would be unable to move a limb. Another characteristic, however, is required. They must 
be always kept sufficiently open for the free passage of air, and it is not easy to see how these 
qualities should be united, as a 
flexible tube will mostly, if abruptly 
bent, as is continually the case with 
the air-tubes of the limbs, lose its 
roundness at the angle, and shut 
off the communication. An India- 
rubber gas-tube is a familiar in- 
stance of this property of flexible 
tubes. 
The difficulty is, however, sur- 
mounted by a simple and yet most 
effectual plan. The tubes are double, 
one within another, and in the in- 
4 ° CATTLE BOT-FLY.—Hypoderma, or Gstrus bovis. a, Fly. b, Larva. c, Pupa. band c, 
terspace a fine but very strong hair- 4s seen trom the under side. (All magnified ; fine at left-hand side natural size of Fly.) 
like thread is closely wound in a 
spiral. It will be seen that, by means of this structure, the tube can be bent in any direction 
without losing its roundness. The long flexible tubes of Turkish pipes are made in a similar 
manner, a spiral wire forming the basis, upon which is sewn the leather and silken outer 
tube—one of the many instances where the 
art of man has been anticipated in the 
animal creation. A third species (Gstrus 
ovis), of which we give also an exact illus- 
tration, is parasitic in the sheep, inhabiting 
the frontal sinus, 7. e., the open space be- 
tween the bones on the forehead and between 
the eyes. 
The  Hippobosecide, 
popularly known under 
the name of Forest-flies, 
deserve a short notice. 
These are round-bodied 
SHEEP BOT-FLY.—(@strus ovis. a, Fly. b, Larva. c, Pupa, as seen insects with legs that can 
from the under side. (Everything magnified.) se 

a b c 

cling with wonderful 
force, and are capable of moving backwards, forwards, and sideways with 
equal swiftness, so that they are not easily captured, even when they do Horse, or FOREST-FLY. 
not take to wing. These insects are mostly found in or near forests, and ea oe equina.- (Mag 
are very annoying to horses and cattle. As may be seen by reference to the Ne 
engraving, their integuments are covered with hair, and are remarkably tough and leathery. 
The various species of Ticks belong to this family, and are closely allied to the Forest-fly. 
Although not included in Mr. Westwood’s list of insects, the Lice are mostly considered 
as belonging to this class, under the name of AprnRa. There are very many species of these 
obnoxious creatures infesting different animals and tribes, and at least three species are found 
upon the human subject. 

(Os) lane) GO \S 
