SOME REPRESENTATIVE INSECTS 387 



face. The larval stage of Culex lasts from one to four weeks, 

 according to the species and to some extent the temperature and 

 food supply. The food during this period consists of minute 

 organisms that are captured by the mandibulate mouth parts. 

 With their third molt these larvae change into active but non- 

 feeding pupae in which the head and thoracic regions are enlarged 

 and the wings and antenna? of the adult are foreshadowed as in 

 the chrysalid of a butterfly (cf. Fig. 195). The pupje must secure 

 air from the surface as do the larvae, but their air tubes are located 

 on the dorsal side of the thorax. They rest at the surface, and 

 when disturbed swim downward by movements of the abdomen, 

 rising to the surface again by virtue of the air contained in the 

 tracheal system, as do the larvae. It is the necessity of obtaining 

 oxygen by this means that enables one to kill mosquito larvae and 

 also the pupae by covering the surface of the water with a thin film 

 of oil which the respiratory tubes are unable to penetrate. After 

 two to five days the pupal skin is cast and the adult emerges. 

 While drying and hardening the wings, it rests upon the sur- 

 face, often supported by the pupal skin. Other genera of 

 mosquitos differ in their habits and in the details of the life 

 cycle, but all undergo the same general structural changes as 

 Culex. 



Hymenoptera. — Like the Coleoptera and the Lepidoptera, the 

 Hymenoptera present a vast array of types, ranging from those of 

 relatively simple hal^its to species with a highly developed social 

 organization like the honey-bee. The examples that follow illus- 

 trate this diversity. A type of hymenopteran that is famihar by 

 its effects upon vegetation, although the adults are not popularly 

 known, includes the gall-flies, or gall-producing insects. Familiar 

 examples are oak-apples, or galls of oak trees, which result from 

 the peculiar habits of these species. The adult female lays her 

 eggs within an oak leaf by means of an ovipositor, which is adapted 

 for piercing such tissues. The larva that hatches from this 

 egg sets up an irritation which causes the abnormal growth of 

 the plant that constitutes the gall. Within this protecting mass 

 the larva develops, feeding upon the plant juices. After pupation 

 and metamorphosis, which usually occur in the fall and winter, 

 the fully formed insect gnaws its way out to liberty. Thus 

 dead oak leaves are often found on the tree or ground, showing the 

 opening from which the gall-fly has escaped. 



