LARVAE OF DIPTERA 153 



two lateral tracheae pass forwards through the body, giving off 

 branches to the tissues as they go, and they communicate with the 

 anterior respiratory openings when such are present. When there are 

 stigmata on the first and last segments of the body (not counting the 

 head) the larva is said to be amphipneustic, while if they are limited 

 to the posterior end the term metapneustic is employed. When 

 stigmata are found along the sides of the body, an uncommon con- 

 dition, the larva is said to be peripneustic. A larva may begin life 

 in the metapneustic stage, and become amphipneustic later. The 

 posterior stigmata are often conspicuous structures, and are usually 

 recognizable with a hand lens. Since they differ in detail in closely 

 allied flies, and often present the only recognizable distinctions between 

 the larvae, they are of use in distinguishing species from one another, 

 both in practical work when breeding for experimental purposes and 

 for systematic classification. 



In many of the aquatic larvae of the Nematocera the structure of 

 the respiratory system is specially adapted to their mode of life. In 

 the larger species of Chironomus the tracheal system is rudimentary 

 and entirely closed, so that gaseous air is not taken into the body, 

 the dissolved oxygen in the water, in all probability, being absorbed 

 by the tracheal gills at the posterior ends of their bodies. These 

 larvae, which live in tunnels at the bottom of pools, and even at 

 great depths in lakes, have in addition a blood-red pigment thus the 

 name blood worm which is believed to act as an oxygen carrier. 



Another nematocerous larva which lives permanently below the 

 surface of water, in this case well aerated water, is that of Simulium. 

 It also has a closed respiratory system. The pupa of this type of 

 aquatic larva has a large number of respiratory filaments, which are 

 well adapted for the absorption of oxygen from water. 



The larvae of the smaller Chironomidae, and of some of the Culicidae 

 (Corethra), which live among weeds at, or near the surface of water, 

 also have closed respiratory systems, but in this case the blood is 

 colourless. Their pupae have respiratory trumpets, and are air breath- 

 ers. The aquatic larvae of Ceratopogon and Culicoides are good 

 examples of this type. 



The larvae of Diptera usually cast their skins three times during 

 the period of their growth, and although the moult is accomplished 

 as a rule with little or no change except an increase in size, certain 

 minor differences are to be noted in the several stages. This is of 

 practical importance and must be taken into account in order to avoid 

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