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Insects have spiracles on their exoskeletons to allow air to enter the trachea. [1] [page needed] In insects, the tracheal tubes primarily deliver oxygen directly into the insects' tissues. The spiracles can be opened and closed in an efficient manner to reduce water loss. This is done by contracting closer muscles surrounding the spiracle.
Although all insects have spiracles, only some arachnids have them. Some spiders such as orb weavers and wolf spiders have spiracles. Ancestrally, spiders have book lungs, not trachea. However, some spiders evolved a tracheal system independently of the tracheal system in insects, which includes independent evolution of the spiracles as well ...
Instead, the insect respiratory system uses a system of internal tubes and sacs through which gases either diffuse or are actively pumped, delivering oxygen directly to tissues that need it via their trachea (element 8 in numbered diagram). Since oxygen is delivered directly, the circulatory system is not used to carry oxygen, and is therefore ...
Depending on the number and position of the tracheal spiracles, the following types of breathing apparatus can be distinguished. apneustic, with absence of stigmas ; metapneustic, with only one pair of stigmata. These are on the abdomen. amphipneustic, with two pairs of stigmas. On pair on the prothorax, one pair on the abdomen.
As insects get larger, gas exchange via spiracles becomes less efficient, and thus the heaviest insect currently weighs less than 100 g. However, with increased atmospheric oxygen levels, as were present in the late Paleozoic , larger insects were possible, such as dragonflies with wingspans of more than two feet (60 cm). [ 59 ]
The forelegs are reduced in the Nymphalidae Diagram of an insect leg. The thorax, which develops from segments 2, 3, and 4 of the larva, consists of three invisibly divided segments, namely prothorax, metathorax, and mesothorax. [11] The organs of insect locomotion – the legs and wings – are borne on the thorax.
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The trachea open through spiracles into this air film, allowing access to oxygen. In many such cases, when the insect dives into the water, it carries a layer of air over parts of its surface, and breathes using this trapped air bubble until it is depleted, then returns to the surface to repeat the process.