40 A BOOK OF INSECTS 



or less firm in texture, and are used to cover and protect 

 the hind-wings when the insect is at rest, as in cock- 

 roaches, grasshoppers, and crickets. Such thickened 

 fore-wings are called tegmina. The horny fore-wings of 

 beetles — a further development along the same lines — are 

 termed wing-cases, or elytra ; and while they appear to 

 play a passive part in flight by acting as balancers, pos- 

 sibly also as gliders or kites, they can hardly be considered 

 as wings at all. 



Among the strange stick and leaf insects of the family 

 Phasmidce the fore-wings are usually very small or entirely 

 wanting ; while the hind-wings are often large and beau- 

 tiful structures, which fold up like a fan beneath a firmer 

 front portion of the wing. There is also a group of small 

 parasitic insects called Stylopidce whose males have the 

 posterior wings well developed, the fore- wings being 

 rudimentary. But in all true flies the hind-wings are 

 suppressed, their place being taken by a pair of stalked 

 knobs, called halteres or balancers, which are thought to 

 be chiefly useful as sense-organs. A few flies, such as 

 the sheep spider-fly, or " ked," often miscalled the sheep- 

 tick, as well as fleas and most other parasitic insects, have 

 no wings. There are, too, other wingless insects scattered 

 throughout the orders ; but most of them are closely 

 related to kinds which have wings. Reference has already 

 been made to species of moths in which the females are 

 wingless, although the males have fully developed wings 

 and are good fliers ; also to the common cockroach, the 

 sexes of which exhibit a similar phenomenon. The 

 aphides, often wingless, produce winged broods at certain 

 seasons of the year, or when a dwindling food-supply 

 makes emigration to other plants desirable. Furthermore, 

 certain insect communities, such as those of ants, comprise 

 wingless " workers " and wing-bearing males and females. 



