17 
(trachew) which run along the sides of the body,and from which smaller tubes convey 
the air in very small volumes to different parts of the body, and so oxygenate the- 
vital fluid in its passage. In the Arachnida this tracheal system is considerably modi- 
fied, respiration being effected among some spiders by ordinary ramified tube-tra- 
chee, and among others in certain sacs or cavities in the abdomen which have been 
called pulmo-branchize from an idea that they partook both of the nature of the 
lungs of the higher animals and the branchiz or gills of fishes; as, however, the 
blood does not penetrate these sacs, but is merely oxygenated in its passage by and. 
round them, while it is being re-collected after use and previous to being sent 
back to the dorsal vessel or heart, these cavities are now believed to be trachez 
localized within a peculiarly furnished sac. Tube-tracheee when examined with 
the microscope are wonderfully beautiful, Traversing as they do the whole 
bodies of insects, they must necessarily be very flexible, and it might be supposed 
that the rapid movements of the insect might sometimes cause them to collapse, 
in which case the circulation of the air would be stopped. Upon examination 
we find that there is a beautiful and simple contrivance which renders this 
impossible. The tubes consist of a double integument between which runs a 
hair-like spirally-twistea fibre, just like the coil of wire which is sometimes put 
inside indian-rubber speaking tubes ; this adds considerably to the elasticity of 
the air vessels, and when these collapse through the movements of the insects, it 
prevents the opposite sides from adhering, and causes them to resume their 
‘tubular form as soon as the pressure is removed. There are generally nine pairs 
of breathing pores or openings through which the air is admitted into the trachee. 
These openings are so constructed that it is impossible for the dust to enter the 
tracheee. At the outside orifice is a corneous plate, and inside that is a hollow 
chamber, and then at the other side of that is another valve. In perfect insects 
nearly all the air enters through the thoracic spiracles. 
“When an insect is preparing itself for flight, the act of respiration resembles 
that of birds under similar circumstances. At the moment of elevating its elytra 
and expanding its wings, which are indeed acts of respiration, the anterior pairs 
of spiracles are opened, and the air rushing into them is extended over the whole 
boay, which by the expansion of the air-bags is enlarged in bulk, and rendered 
of less specific gravity ; so that when the spiracles are closed, at the instant the insect 
endeavors to make the first stroke with, and raise itself upon its wings, it is enabled 
to rise in the air, and sustain a long and powerful flight with but little muscular 
exertion.” In the pupal and larval state respiration is performed almost equally 
by all the spiracles; but in the imago almost all the air enters by those in and near 
the thorax, so that, generally, a pinch under the thorax will kill most soft-bodied 
insects by suffocating them. 
Perhaps no sub-order of insects presents more examples of wonderful. 
instinct and exquisite adaptation of their forms and habits to required ends than 
