“This method, similar to that employed by the English physiologist, Hutchinson,[[71]] is valuable, because it enables us, with a little practice, to investigate readily the respiratory movements of very small arthropods, such as flies or lady-birds. It has this advantage over all others, that it leaves no room for errors of interpretation.”
“Not satisfied with mere observation by such means as these, of the respiratory movements of insects, the writer has also studied the muscles concerned, and, in common with other physiologists (Faivre, Barlow, Luchsinger, Dönhoff, and Langendorff), has examined the action of the various nervous centres upon the respiratory organs. The result at which he has arrived may be summarized as follows:—
Fig. 415.—Muscles of right half of the abdomen of Forficula auricularia: A, a, longitudinal tergal and sternal muscles; D, E, oblique muscles; a (in upper figure) vertical expirator muscles.
“1. There is no close relation between the character of the respiratory movements of an insect and its systematic position. Respiratory movements are similar only when the arrangement of the abdominal segments, and especially when the disposition of the attached muscles, are almost identical. Thus, for example, the respiratory movements of the cockroach are different from those of other Orthoptera, resembling those of the heteropterous Hemiptera. Those of the Trichoptera are like those of the aculeate Hymenoptera, while the Locustidæ ally themselves in respect to these movements with the Neuroptera and Lepidoptera.
“2. The respiratory movements of insects, when at rest, are localized in the abdomen. As graphically stated by Graber, in insects the chest is placed at the hinder end of the body. If thoracic respiratory movements exist, they do not depend on the action of special muscles.
“3. In most cases the thoracic segments do not share in the respiratory movements of an insect at rest. The respiratory displacements of the posterior segments of the thorax are, however, less rare than Rathke believed. Plateau has observed them in certain Coleoptera (Staphylinus, Chlorophanus, Corymbites), and they are more feebly manifested in Hydrophilus, Carabus, and Tenebrio. Among the singular exceptions to this rule is the cockroach (Periplaneta orientalis), in which the terga of the meso- and metathoracic segments perform movements exactly opposite in direction to those of the abdomen (Fig. 419).
Fig. 416.—Muscles of the left half of abdomen of Staphylinus olens; A, B, longitudinal dorsal muscles; D, E, oblique fascia; a, longitudinal sternal muscles; d, respiratory muscles (vertical expirators).
“4. Leaving out of account all details and all exceptions, the respiratory movements of insects may be said to consist of the alternate contraction and recovery of the figure of the abdomen in two dimensions, viz. vertical and transverse. During expiration both diameters are reduced, while during inspiration they revert to their previous amounts. The transverse expiratory contraction is often slight, and may be imperceptible. On the other hand, the vertical expiratory contraction is never absent, and usually marked. In the cockroach (P. orientalis) it amounts to one-eighth of the depth of the abdomen (between segments 2 and 3); in Eristalis tenax to one-ninth (at the 2d segment).