PHYLUM ARTHROPODA 



311 



all of ectodermal origin except the optic nerve itself. Compound eyes 

 are widespread in the arthropods, being found in the trilobites, crus- 

 taceans, king crabs and insects. They are lacking in the other groups of 

 living arachnomorphs and labiates. 



Most insects also have ocelli, small eyes on the top of the head. 

 Typically three of these are arranged in a triangle. Each ocellus (Fig. 

 16.21) is a group of retinuli, comparable to the lower portions of om- 

 matidia, underlying a single large lens. In most insects the retinuli lie 

 too close to the lens for an image to be formed. The function of these 

 eyes is not understood. They are believed to monitor light intensity and 

 to influence the insect's general level of activity rather than to provide 

 spatial information on light distribution. In Periplaneta the ocelli are 

 degenerate. 



Organs of touch are special tactile bristles scattered over the body 

 and especially prominent on the antennae, palps and cerci. On the cerci 

 they vibrate in response to wind or low sounds. Smell and taste are 

 mediated by chemoreceptors clustered on these same organs. The chemo- 

 receptors are projecting cones with a very thin exoskeleton kept moist 

 by glandular secretions. Those on the antennae and cerci are olfactory, 

 those on the palp are gustatory. The distinction between smell and taste 

 depends upon whether the chemical sensed is airborne or dissolved in 

 liquid. 



The tracheal tubes found in all labiates are a respiratory system of 

 air ducts leading in from the spiracles to all the tissues of the body (Fig. 

 16.22). The larger tubes anastomose, forming a network from which 

 smaller tubes ramify. Each is a cylinder of epidermal tissue lined with a 

 thin layer of chitin thickened spirally to provide strength. The smallest 

 branches end blindly in tracheoles (Fig. 16.22), minute branching tun- 

 nels within the cytoplasm of end cells. End cells are applied closely to 

 the surfaces of other cells. The cockroach flushes air in and out of the 

 system by respiratory movements, or breathing, in which the abdomen is 

 alternately flattened and relaxed by the contraction and relaxation of 

 stout vertical muscles within it. 



eal lens 



Epidermis 

 Rhabdome. 



Pi6m<zjrrt cell 

 R<ztinula_cell 



Nerve fibers 



Figure 16.21. Diagrammatic section through an insect ocellus. (After Comstock.) 



