Exchange of respiratory gases in insects and allied terrestrial arthropods takes a very different form from that found in other animals. The anatomy of most contains a system of tubules (tracheal system) dedicated directly to the tasks of bringing oxygen to the tissues and carrying off carbon dioxide and other waste gases. The blood plays no significant role in this process except in very small, immature forms that live in damp conditions and aquatics with blood-filled gills. The tracheae open to the outside through segmen-tally arranged pores, the spiracles, which generally have a closing device to keep water loss to a minimum. Large tubes run inward from the spiracles and branch profusely, often interconnecting with sacs or other tubules and terminating finally in minute blind endings (tracheoles) directly on the cells. Derived from integumentary epidermal cells, the entire system, except the tracheoles, is lined with cuticle that has a circular ringed structure for strength against collapse.

The rates of diffusion of oxygen and carbon dioxide are sufficient to allow these gases to passively reach all tissues, but ventilatory movements are necessary in large and very active forms. This is accomplished by abdominal compression, contraction, and other muscular movements. The length of the diffusion path, however, is a factor limiting the size of insects, in particular, those with the bulky muscle masses needed for flight.

Spiders have a tracheal system in the abdomen only, including a variety of modifications, among them "sieve trachea," which are large trunks from the ends of which originate numerous individual fine tracheae. Many also possess unique respiratory structures called "book lungs," which are lamellate, trachealike plates extending into the body cavity. Blood flows between the plates, exchanging molecules with the chambers of the tracheoid tubules, thus functioning much like a vertebrate lung.

The mechanisms of external respiratory adaptations in aquatic insects (Miller 1974) go in a great variety of directions. They rely on tapping atmospheric air, or extraction of dissolved oxygen from the surrounding liquid, or combinations of both. Among the former, most are often associated air stores of one kind or another. The tracheal system itself may have sacs or enlargements to accommodate air supplies, or bubbles may be carried beneath the wings or held onto the general body surface by hairs or other extensions of the integument. Prevented from collapse by these extensions, these air bubbles act as "physical gills," oxygen and carbon dioxide passing in and out of them through their surface, which acts like a membrane ("plastron respiration"). Spiracles communicating with the bubbles tap the air store and can also function normally should the water dry up or the animal emerge to assume a terrestrial phase of existence.

Species utilizing atmospheric air must come to the surface from time to time to restore their gaseous provisions, although some, such as certain mosquito larvae, may stay below for very long periods of time, tapping air carried in the vessels of aquatic plants.

Small aquatic insects may employ the general cuticle as a gill. Large types have other forms of gill structures, expansive plates or fingerlike extensions filled with blood, or a rich tracheal network to carry on gas exchange. Respiration in many endoparasitic types relies on similar mechanisms, their lives being spent in a liquid ambience for long periods.

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