1921] Muttkowski: Respiration of Insects 155: 
As in the incinerations of insects for copper, crayfish blood was used 
as a control, with identical results. It seems then that in the latter the 
hemocyanin serves as a carrier for both oxygen and carbon dioxide, 
analogous to hemoglobin; and the presumption is that the same applies 
to insect blood. 
Among other items, the Malpighian tubules of insects were studied 
as a possible exit for Carbon dioxide and carbonates. A number of 
experiments were made with the reagents mentioned on a variety of 
tubules, including those of bumblebees, larval and adult Dytiscus, 
Anax and Aeshna larvee, Belostoma, Enallagma, and others. In sum- 
mary, ten of eighty-one experiments showed positive results, the 
remaining seventy-one negative or doubtful. Even with a liberal 
allowance for error, these results are too insufficient to permit of any 
conclusions. 
Elsewhere I have offered the suggestion (L. c., p. 93) that at least 
in aquatic larve the external chitin membrane may serve for the passage 
of carbon dioxide. This suggestion was based on some experiments on 
the permeability of the chitin membrane to gases, performed some years 
ago. However, this might apply to aquatic insects, it would not cover 
the conditions met in terrestrial stages. 
IV. THe Rote oF InsEct Bioop. 
Hitherto our interpretation of the role of insect blood has been that 
the blood transports food, glandular products, waste, and pigments, 
and that it takes no part in the process of respiration. From the fore- 
going experiments it is evident that this interpretation must be amended 
to include respiration. 
That the blood is active in transporting foods-can be readily shown 
in testing fed and starved insects. After a meal the blood shows a 
large supply of nitrates, nitrites, and phosphates, the latter depending 
on the quality of food, whether rich or poor in phosphates. Ina starved 
specimen, not fed for three or more days, none or only traces of these 
substances are indicated. 
Glandular products include corpuscles, enzymes, some form of 
coagulin, and pigments. The pigments are carried passively in the 
blood stream, to be elaborated during the pupal period. The enzymes 
present are chiefly of the oxidative, histolytic, and autolytic types. 
Here also should be placed hemocyanin and hemoglobin, the respiratory 
proteins. In what tissues or organs these are elaborated has not been 
ascertained. Coagulin in some form must also be present, as the blood 
of insects clots readily on exposure to atmospheric air. This clot is 
generally black, but brown in adult Dytiscus. Whether this coagulin 
originates as a zymogen of the prothrombin type, forming thrombin, 
and combining with fibrinogen to form fibrin, as postulated for mammal- 
ian blood, is unknown. 
Of gases the blood contains both oxygen and carbon dioxide, some 
probably dissolved in the blood serum, the major portion held by the 
hemocyanin present. 
