THE PHYLA ARTHROPODA AND ONYCHOPHORA 



blood flows posteriorly through the hemocoelic spaces, bathing the tissues, un- 

 til it again reaches the abdomen, passes through the dorsal diaphragm, and 

 re-enters the pericardial sinus and the heart. There is a variety of colorless, 

 amoeboid cells in the blood of insects, but nothing comparable to the erythro- 

 cytes of vertebrates; the blood contains no pigments involved in the transport 

 of oxygen. 



The Nervous System and Responsiveness. The nervous system of the locust 

 is arranged according to the typical arthropod plan, as described for the cray- 

 fish. There is a compound ganglionic mass in the head, termed the "brain" 

 or supraesophageal ganglion,- a pair of circumesophageal connectives; a sub- 

 esophageal ganglion; and a ventral chain of paired, segmental ganglia with 

 their connectives and radiating nerves (Fig. 15.18). The brain lies in the 

 head between the compound eyes. Although it is obviously a compound 

 structure produced by fusion of segmental ganglia, the exact number of such 

 ganglia included cannot be determined. Large optic nerves pass into the 

 brain from the compound eyes, smaller nerves from the ocelli, the antennae, 

 and the labrum. In addition, small nerves extend ventrally to the frontal 

 ganglion, from which the visceral or sympathetic nerve leads to the anterior 

 part of the digestive tract. The circumesophageal connectives pass around 

 the digestive tract and join the subesophageal ganglion. This also has a com- 

 pound origin, apparently being formed during development by the coalescence 

 of three pairs of segmental ganglia. From the subesophageal ganglion nerves 

 radiate to the mandibles, the maxillae, and the labium. The thorax contains 

 three paired ganglia, corresponding to the three thoracic somites. But the 

 posterior ganglion is comparatively large, and its nerves are distributed in a 

 way that indicates its formation by fusion of the third thoracic and first ab- 

 dominal ganglia. There are five ganglionic masses in the abdomen, which 

 again represent fusions, particularly at the posterior end of the nerve cord. 

 The adult locust, therefore, has fewer ganglia than somites, but in the embryo 

 there is a ganglion for each somite, as in the less highly specialized arthropods. 



Like the majority of other insects, locusts are well equipped with sense 

 organs. Small receptors for tactile stimuli are widely distributed over 

 the surface of the body, and they are concentrated in especially sensitive areas, 

 such as the antennae, the cerci at the tip of the abdomen, some of the mouth 

 parts, and the distal segments of the legs. Olfactory stimuli also affect the basal 

 parts of the antennae. Some insects are able to respond to very slight olfac- 

 tory stimuli. For example, the males of certain species of moths will fly up- 

 wind from considerable distances in response to odors emitted by the scent 

 glands of the females. Gustatory stimuli are perceived by taste organs on the 

 mouth parts of the locust. The ocelli are sensitive to light but are probably 

 incapable of forming images. When the compound eyes of a locust are 

 covered with black paint, the insect will not react to moving objects by leap- 

 ing away, but it will find its way out of a box in which there is only one small 

 opening to admit light. When the ocelli as well as the compound eyes are 

 covered, the animal escapes from the box only by chance. It has been shown 



451 



