226 



THE RISE OF ANIMAL LIFE 



turn, and then to the outside through the 

 anus. 



Circulatory system. The body cavity of 

 the grasshopper is the haemocoel, or blood 

 cavity, not the coelom, as was the case in 

 the earthworm. This is formed by a contin- 

 ued expansion of the vascular system until 

 the entire coelom is obliterated, with the 

 exception of the cavities of the gonads. The 

 cavity is filled with a colorless blood which 

 contains only leucocytes. Since the blood 

 does not function as a conveyor of oxygen, 

 it has no oxygen-carrying pigment as the 

 blood of most other animals has. The blood 

 is kept in motion by the action of the dorsal 

 tube-like heart, composed of a number of 

 chambers into which small ostia open ( Fig. 

 11-21). The heart is surrounded by a peri- 

 cardial sinus, which holds the blood before 

 it enters the heart, much the same as in the 

 crayfish. As the heart beats, blood moves 

 forward and out into the haemocoel again. 

 The heart has no occasion to be as active 

 an organ as it is in many other animals, 

 since respiration is carried on in another 

 fashion. 



Breathing system. The respiratory system 

 of insects is unique in the animal world. It 

 seems strange that it should have appeared 

 in this one group of animals and nowhere 

 else. Air, with its oxygen, is carried directly 

 to the cells through a system of tubules 

 called trachea (Fig. 11-22). This very com- 

 plex system consists of tiny tubes which 

 must remain distended so that the air can 

 pass freely in and out of them. Small chitin- 

 ous spiral threads give support to the tu- 

 bules and control their diameter. There are 

 several openings into the trachea along the 

 thoracic and abdominal walls; these are 

 called spiracles. A valve covers the opening 

 so that the spiracle can be opened and 

 closed during the breathing process (Fig. 

 11-22). Leading in from the spiracles, the 

 tubules become smaller and smaller until 

 they are as small as capillaries and lie di- 

 rectly against the cells, supplying them 

 with oxygen and carrying away the ex- 



creted carbon dioxide. The grasshopper 

 also possesses several large air sacs which 

 may be contracted to aid the movement of 

 air through the many small tubules. The 

 grasshopper contracts and enlarges its 

 body, particularly the abdominal region to 

 facilitate the air flow and the anterior spi- 

 racles open and close alternately with the 

 posterior spiracles, so that the air makes 

 a one-way passage. 



The nervous system. While the grass- 

 hopper has many of tlie sense organs that 

 the crayfish possesses, it lives in an air en- 

 vironment and therefore needs somewhat 

 different methods of maintaining contact 

 with its outside world. For example, the 

 eyes and the tactile hairs which cover the 

 various parts of its body resemble those of 

 the crayfish, and it has organs of chemi- 

 cal sense on its antennae and mouth parts. 

 However, the grasshopper and many other 

 insects have the means of making and re- 

 ceiving sound vibrations. In the grasshop- 

 per the organ for hearing is on the first 

 segment of the abdomen. In several re- 

 spects this resembles the ears of higher 

 vertebrates, in that it is composed of a 

 stretched membrane, the tympanum, to 

 which is attached a slender process that is 

 connected to a nerve. The animal makes its 

 characteristic clacking sound by rubbing 

 its roughened hind tibias against a wing 

 vein. 



As a result of the fusion of three head 

 segments, the grasshopper possesses a 

 rather large brain. Nerves extend directly 

 from the brain to the eyes, the antennae, 

 and to a ganglionated cord running through 

 the body. The first ganglion in tlie cord, 

 the subesophageal ganglion, formed by two 

 great nerves or connectives proceeding from 

 the brain around the esophagus, is fol- 

 lowed by three large ganglia in the thorax 

 and five in the abdomen, where some fu- 

 sion has gone on. Nerves go out to all parts 

 of each segment and to the legs and wings, 

 and there is also a fine network of nerves 

 beneath the epidermis. The nervous system 



