Caddis larvae which bear cases ventilate their gills by undulatory movements of the abdomen. These respiratory movements produce a stream of water flowing out through the posterior opening of the case. In well-aerated water the movements are intermittent, periods of undulation alternating with pauses. In poorly aerated water the pauses become shorter.
The only study of the environmental control of these respiratory movements by caddis larvae is that of van Dam (1938). This worker studied the larvae of Phryganea grandis L. which had been removed from their cases and confined in celluloid tubes open at each end. He found that raising the temperature shortened and finally abolished the pauses between bursts of undulation. The same result was obtained when the oxygen content of the outside water was lowered by bubbling nitrogen through it. When aerated water was passed directly through the celluloid tubes, respiratory movements ceased. Carbon dioxide caused no acceleration in the movements. It appears then that in nature a rise in temperature or a decrease in dissolved oxygen augments the amount of ventilation of the gills. Van Dam observed that the amplitude of the respiratory movements is also thus increased. He saw, too, that crawling decreases the pauses, doubtless through oxygen-lack. He also observed several strong and rapid ventilation movements after defaecation, an adaptation to wash faeces out of the case.
These observations are very interesting, but they are inadequate since no measurements were made of the oxygen content of the water, nor is it stated how many individuals were studied. Moreover, the tubes in which the animals were confined do not imitate conditions in nature. We therefore thought it worth while to make a further study of the question. We used the larvae of Limnophilus flavicornis Fabricius, from the Long Water at Hampton Court. Two days before an experiment, animals were removed from their cases and given pieces of cellophane of about the same size as the material used in nature for case-building. With this substance the larvae constructed transparent cases. For observation the animals were placed in stoppered glass bottles of water, immersed in a glass tank of water at 20° C. The respiratory movements of each animal during a period of 8 min. were recorded on a smoked drum with a lever actuated electrically by a tapping key. This gave the time of each movement, but unfortunately not the amplitude. The number of beats per minute was later computed.
In each of 25 experiments on the influence of oxygen, a record was first made of the respiratory movements in fully aerated water, and then this water was siphoned off from the bottle and replaced by water of a low oxygen content, previously prepared by bubbling nitrogen through it. After a second record of the movements, the stopper was again removed and a sample of the water was analysed for dissolved oxygen by the micro-Winkler technique of Fox & Wingfield (1938).* In ten of the experiments a third record was made at a lower oxygen content. We found irregular pauses in the ventilation by the larvae in air-saturated water, and we confirmed van Dam’s observations of the abolition of pauses and the increased amplitude of movement in poorly aerated water, and also of the special movements after defaecation.
Our results are given in Fig. 1, where the oxygen content of the water is expressed as a percentage of air saturation, which, at 20° C., means an oxygen concentration of 6·4 ml./l. The mean number of movements per minute of each of sixty records is entered in the figure. It is clear from the figure that a low oxygen content of the water increases the rate of respiratory movements. The correlation coefficient between percentage air saturation (x) and movements per minute (y) is r= –0·841 (P <0·001). (A less high correlation coefficient is obtained between percentage air saturation and the logarithms of movements per minute.) The regression line of y on x is given by y = 74·61 – 0·527x, and has been inserted in Fig. 1.
We have confirmed the observation of van Dam that carbon dioxide does not accelerate the respiratory movements. The experiments were made in fully aerated water to which small quantities of water saturated with carbon dioxide had been added to reduce the pH to definite measured values. In thirty-three records on the drum of respiratory movements at 20° C., the mean rate of movements, namely 20 per minute, was the same at pH 8·0 and 7·5, whilst at pH 6·7 the rate fell to 13 and at pH 6·4 to zero. The effect of carbonic acid at the low pH values was narcotic, for the normal rate was resumed on return to aerated water.
A quantitative study has been made of the effects of dissolved oxygen and carbon dioxide on the respiratory movements of the larva of Limnophilus flavicornis. A diminution in oxygen accelerates the movements but an increase in carbon dioxide has no such effect.
The analyses were kindly made for us by Miss Barbara M. Gilchrist.