264 Comparative Animal Physiology 



dioxide may be due to its absorption by the exoskeleton, as pointed out by 

 Peters,-'""^ who found it is necessary to coat the animal with collodion to get 

 consistent results; presumably no harmful effects are produced by the sealing. 

 Jordan and Ouittart'''" describe a kind of Cheyne-Stokes respiratory pattern 

 resulting from treatment with excess CO-. After an interval of apnea, there 

 ensues a period of discontinuous hyperventilation, followed by eupnea despite 

 continual exposure to 10 per cent carbon dioxide. The evidence indicates a 

 stimulating action of CO- in the crayfish, but, owing both to its possible fixa- 

 tion in the shell and to its solubility in sea water, this stimulant may be less 

 effective than is oxygen lack. 



In Eriocheir sinensis both HCl and COo in mild excess inhibit ventilation, 

 but in considerable amounts stimulate respiration. ^^^ After removal of the 

 antennal appendages the respiratory inhibition is lost, suggesting the presence 

 of carbon dioxide receptors on the antennae. 



The movement of the pleopods on several isopod and amphipod crustaceans, 

 in response to respiratory stimulants, seem to indicate a rather pronounced 

 generic difference in these forms (Table 47). This may be correlated in some 

 way with habitat; the terrestrial Ligia, for example, responds neither to CO2 

 excess nor to Oo reduction, but does show a slowing of pleopod rhythm when 

 the oxygen tension is increased. ^^"^ 



Breathing in Insects. These organisms have received the major share of 

 attention in studies of invertebrate respiration, largely because of their econom- 

 ic value but also because of the interest in their unique breathing mechanism. 

 A number of excellent reviews are available for consultation of the very ex- 

 tensive literature in this field dating as far back as Vauquelin's stimulating 

 work on the Orthoptera in 1792.^'^' 30«. 366, 368 



Briefly, two kinds of movement may be distinguished: rhythmic pumping 

 motions which involve the abdomen and part of the thorax, and occasional 

 opening and closing of the spiracles. Expiration is an active process; inspiration 

 is either active or passive. All these movements are coordinated by respiratory 

 centers— "primary" segmental centers and "secondary" higher-order centers in 

 the prothorax.-^-'^^' ^'^^ A complete elaboration of orthopteran respiration was 

 presented by Lee,---- --■' who clearly demonstrated the direction of air flow 



and the correlation of ventilation movements with spiracular patency and 

 occlusion. Further ingenious experiments on grasshoppers by McCutcheon 

 have shown the presence not only of an inspiratory and an expiratory phase, 

 but of a compressatory phase as well, a relatively long interval during which 

 intratracheal pressures may increase from 2 to 30 mm. Hg above ambient 

 pressure.-^'"^ In the grasshopper the anterior spiracles (the two pairs of thoracic 

 and the first two pairs of abdominal) are inspiratory, while the posterior 

 (abdominal) apertures are expiratory. Valvular mechanisms direct the flow, 

 and central reflexes control the synchronization of movement. In abnormal 

 cases, e.g., in blockage of the anterior spiracles, air both enters and leaves by 

 way of the abdominal tracheae. Owing to loss of vital water through open 

 tracheae, the normal preferred condition of the spiracles is a closed one. 

 1 lowcver, to permit gas exchange the spiracles must either remain partially 

 open, as in the flea, or open and close alternately, as in the grasshopper. That 

 this pattern of directional air flow as described for the grasshopper is only one 

 of many is emphasized by the gas exchange in the larval dytiscid, Cyhister, 



