DIPTERA 257 



segments are fused. The larva is apodous (Fig. 191), and the pupa 

 may be obtect (though often capable of movement) or coarctate. 

 The gnats or mosquitoes lay their eggs on the surface of water, 

 different species requiring different ecological conditions of size 

 of pond, temperature, hydrogen ion concentration, salinity, and 

 so on. The larvae, although legless, are very active, and swim 

 beneath the surface by a wriggling movement of the whole body ; 

 they feed on minute particles in the water by means of brush-like 

 structures which continually sweep a current of water into the 



Fig. 189. — A ventral view of the head of a fully grown larva of the mosquito 

 Anopheles maculipennis. — From Nuttall and Shipley. 



b., Brush with which fcod is swept from the surface film of the water (Culex larvas, hanging with the head 

 down, collect from a lower stratum); c, antenna: d., palp of maxilla ; ;'., stout hairs which arrange 

 the brush ; k., teeth of mandible ; m., hooked hairs at edge of maxilla ; />., a median tuft of hairs ; 

 q., a median structure known as the metastoma ; r., rim of head. 



mouth and are combed by the mandibles and maxillae (Fig. 189), 

 and breathe air which they obtain at the surface. Most of the 

 spiracles are closed, but the two main tracheal tubes open on a 

 projection of the eighth segment of the abdomen called the respir- 

 atory siphon. This is closed with flaps when the creature is under 

 the water, and when it comes to the surface they spread out on the 

 surface film, expose the tracheal openings to the air, and hang the 

 insect from the film. Water, which has high surface tension, will 

 not enter the narrow tube, but oil, with low surface tension, will- 

 as can easily be observed under the microscope ; this is the 

 principle of the method of attacking the carriers of malaria 

 (p. 76) by spraying paraffin on the water. After four months 



