Class Insecta 531 



to the polarization of light. It has been shown that bees are able to find 

 their way by responding to the polarization of the sun's rays. 



Tactile organs are chiefly small, innervated hairs which are scat- 

 tered all over the body surface and usually concentrated on the antennae. 



Chemoreceptors seem to be located chiefly on the antennae, and 

 they play a very important role in the life of the insect. Many rely 

 upon this sense to discover mates, food, and places to lay eggs. Among 

 some moths, the female possesses special scent glands by which the 

 males may be attracted from long distances. Other chemoreceptors are 

 located in various areas of the insect body : butterflies, for example, 

 have taste receptors in their tarsi. 



HABIT VARIATIONS 



Even more striking than the structural variations are the many 

 diverse habits which are found among the insects. These habits range 

 from the apparent aimless wanderings of some insects in search of food 

 and mates to the complex insect societies as represented by the social 

 insects. No matter how highly evolved such behavior patterns may be, 

 they represent an inherited pattern which in most cases is rigidly fixed. 

 Many of the more successful insect societies simply fail to survive 

 when any factor of the environment is altered. 



Parasitic Insects. — While the vast majority of insects are free- 

 living, many parasitic forms are known. The degree of parasitism varies 

 from those which are parasitic throughout their entire life cycle to those 

 which depend upon parasitism for a portion of their development. 

 Among the first are such ectoparasites as fleas or lice. These highly 

 modified insects live among the hairs of mammals or the feathers of 

 birds and feed on blood or portions of the skin. 



Among those insects which are parasitic only for a portion of the 

 life cycle are the chalcid wasps. The larval forms of these insects 

 develop within the tissues of the larvae of other insects. Eventually 

 they consume the entire host, but the adult of these parasitic forms is 

 a free-living animal. Even more specialized in this same type of 

 parasitism is the digger wasp. The adult female of Discolia duhia, for 

 example, digs through the soil until it encounters a white beetle grub. 

 It quickly paralyzes this victim by stinging it, and then deposits a single 

 egg. This tgg soon hatches into a larva which starts consuming the 

 paralyzed grub. Within a few wrecks, the wasp larva has finished its 

 meal and forms a pupal case in which it overwinters. 



