The number of spiracles is greatly reduced; in the adult a pair is present on the mesothorax, sometimes also a pair on the metathorax, and there is always a pair on the first and another pair on the eighth abdominal segment.
These spiracles, according to Hinds, are remarkable honeycomb-like structures, and perforations to the tracheal tubes have not been demonstrated.
The young insect resembles its parent in most points, but the head is disproportionately large; the anterior abdominal spiracles are on the second segment instead of on the first, and the foot has only a single segment.
They breathe by piercing the surface film with the tail, where a pair of spiracles are situated.
After eating the contents of the egg, the larva moults and becomes a fleshy grub with short legs and with paired spiracles close to the dorsal region, so that, as it floats in and devours the honey, it obtains a supply of air.
Specially characteristic of the class, however, is the presence of a complex system of air-tubes (tracheae) for respiration, usually opening to the exterior by a series of paired spiracles on certain of the body segments.
Atmospheric air gains access to the air-tubes through paired spiracles or stigmata, which usually occur laterally on most of the body-segments.
These spiracles have firm chitinous edges, and can be closed by valves moved by special muscles.
When the spiracles are open and the body contracts, air is expired.
The subsequent expansion of the body causes fresh air to enter the tracheal system, and if the spiracles be then closed and the body again contracted, this air is driven to the finest branches of the air-tubes, where a direct oxygenation of the tissues takes place.
The air-tubes, like the food-canal, are formed by invaginations of the ectoderm, which arise close to the developing appendages, the rudimentary spiracles appearing soon after the budding limbs.
The embryonic ectoderm of an insect consists of a layer of cells forming a continuous structure, the orifices in it - mouth, spiracles, anus and terminal portions of the genital ducts - being invaginations of the outer wall.
B, spiracle on prothorax; c, protruded head region; d, tail-end with functional spiracles; e, f, head region with mouth hooks protruded; g, hooks retracted; h, eggs.
The series of paired spiracles on most of the trunk-segments is well displayed, as a rule, in terrestrial larvae - caterpillars and the grubs of most beetles, for example.
In many aquatic larvae we find that all the spiracles are closed up, or become functionless, except a pair at the hinder end which are associated with some arrangement - such as the valvular flaps of the gnat larva or the telescopic " tail " of the drone-fly larva - for piercing the surface film and drawing periodical supplies of atmospheric air.
A similar restriction of the functional spiracles to the tail-end (fig.
Respiratory organs tracheal, opening by a pair of spiracles in the prosoma above the base of the fifth appendage on IV III I composed, at least in many cases, of eleven somites, the 1 1 th somite very small, often hidden within the loth.
- Opisthosoma consisting of ten segments defined by integumental grooves, each of the anterior four of these furnished with a single pair of dorsally-placed spiracles or tracheal stigmata.
Of the abdominal pleura) which when pressed together form a tube whose point can pierce the surface film and convey air to the hindmost spiracles which are alone functional in the adult.
The Notonectidae breathe mostly through the thoracic spiracles; the air is conveyed to these from the tail-end, which is brought to the surface, along a kind of tunnel formed by overlapping hairs.
The air-tube system is developed in varying degree in different bristle-tails, the number of pairs of spiracles being three (Campodea), nine (Machilis), ten (Lepisma), or eleven (Japyx).