Some features of the respiratory system of Hydrocyrius columbiae Spin. (Belostomatidae, Hemiptera)

Abstract

The fine structure of the sieve-plates covering the spiracles of Hydrocyrius columbiae has been shown to consist of stout trabeculae bearing a system of minutely branching struts which ultimately form the boundaries of pores about 0·5 μ 2 in area in spiracles 1, 2, and 4–10, and about 3 μ 2 in spiracle 3. Measurements of the airflow through the spiracles under different pressures show it to be about a hundred times faster through spiracle 3 than through any other. Spiracle 3, facing posteriorly into the sub-hemelytral airstore, communicates with the principal tracheae to the flight muscles. While the intact bug can withstand pressures of at least 220 cm Hg without ill effect, after exposure of the spiracles by removing overlying cuticular structures, the sieve-plates permit water to enter the tracheae under about 150 cm Hg. The sieve-plates do not make any appreciable reduction in the rate of water loss when the bug is in air. A ventral hair-pile, consisting of small hooked hairs and long hairs terminating in flattened blades, functions mainly to allow the ventrally-placed abdominal spiracles to communicate with the sub-hemelytral airstore via three bridges. All but the first pair of spiracles are in contact with the airstore which communicates with the atmosphere through a pair of retractile siphons. The siphons are able to curve dorsally by means of muscles and are able to pierce the meniscus when the bug rests horizontally at the surface. Comparisons are made with some other Belostomatidae and with the larva in which a ventrally-placed airstore communicates with all the functional spiracles. Spiracle 3 of the larva does not differ in structure from the other spiracles.