Chap. 30 ARTHROPODS INSECTS, SPIDERS, AND ALLIES 611 



cell forming its socket, and the sensory nerve cell. The tip of this is in contact 

 with the base of the hair exposed to the changes in pressure that it communi- 

 cates to the nerve centers. Such tactile organs are abundant on the antennae 

 and ovipositors of grasshoppers. 



HEARING. In the red-legged, the lubber, and other common grasshoppers 

 there is an eardrum on each side of the first abdominal segment (Fig. 30.10). 

 In some species it is on the front legs. Comparatively few insects, among them 

 grasshoppers, crickets, and cicadas have these eardrums. In the common short- 

 horned grasshoppers, the eardrum is a thin cuticular drum fully exposed on 

 the outside and closely associated with a group of peculiar sensory cells. 



CHEMICAL SENSES — SMELL AND TASTE. Smell and taste are both chemical 

 senses and not easy to distinguish. The chitin that covers these sensory cells is 

 so thin that chemical substances can easily penetrate it. Chemical sense organs 

 are often on minute knobs; others are in pits. Smell is located chiefly in the 

 antennae and the palps. Grasshoppers are sensititve to temperature all over 

 their bodies. They have sharp temperature preferences and as far as possible 

 choose their own private climates in protected sunny nooks. 



COMPOUND EYES. Thcsc cyes are immovable, set well over to the side of 

 the head and a diflferent object is seen through each one at the same time. They 

 are composed of single eyes, usually thousands of them, through which pieces 

 of an object appear in mosaic vision as in the similar eyes of crayfishes. 

 Processes from the light sensitive cells of the eye continue through the optic 

 nerve and are associated with nerve cells in the brain. As in all animals, the 

 interpretation of vision occurs in the brain. That insects do interpret what 

 they see is evident from experiments with honeybees. On the surface of a 

 compound eye its units appear as many six-sided areas, each one a transparent 

 lenslike cornea. Directly beneath this is the crystalline cone composed of 

 crystal clear cells. This in turn rests upon the light receptors or retinal cells that 

 are sensitive to light on the sides meeting in the center of a peculiar rosette 

 (rhabdom). A process extends from each of the light receptor cells and 

 together they form the optic nerve connecting the eye with the brain. A curtain 

 of pigment cells keeps the light that falls on one unit from striking any other. 

 As more or less light falls upon the eye, granules in the pigment cells move to 

 different positions. This shuts out or lets in the light upon the retinal cells just 

 as the iris of the human eye curtains the light sensitive retina. 



Reproduction. The sexes are separate in all insects. In most species they 

 are readily distinguishable by the external sexual structures on the abdomen. 

 There are two testes in which the sperm cells develop. The latter are dis- 

 charged into two tubes (the vasa deferentia) which unite to form the ejacu- 

 latory duct extending through the penis, the organ by which the sperm cells are 

 transferred into the female reproductive passage during mating. Each ovary 

 consists of a group of egg tubules within which the eggs develop (Fig. 30.14). 



