Araneus diadematus, Clerck, 1757

Hill, David Edwin, 2023, Studies on spider respiration and circulation by Victor Willem, Peckhamia 304 (1), pp. 1-46 : 5-9

1. Araneus diadematus

Having familiarized myself with F. Plateau's working methods and reasoning, I thought it essential to pay attention to phenomena he had overlooked and, first of all, to elucidate the nature and origin of movements that this meticulous observer had incidentally noticed in the animals he studied: rather regular and rapid oscillations (130 or 147 per minute) of the abdomen, vertical and of very small amplitude (up to one-sixth of a millimeter), sometimes accompanied by similar movements of the pedipalps or a leg.

The season led me to begin with Araneus , whose abdomen, as Plateau said (p. 344), "shows oscillations so slight that it would be very difficult to determine their amplitude." I did not want to use Plateau's "projection method," which seemed too simplistic for the present case [A magic lantern projected an enlarged silhouette of the animal onto a screen.]: I examined various regions of the body surface under the microscope, with a magnification of 30-70 diameters, allowing me to observe very slight deformations.

The animal is held on the edge of a suitably cut piece of cork, to which its legs are also attached, using very fine pins that embrace the thorax. Alternatively, a ligature, encompassing all the legs brought dorsally, fixes it to the end of a needle [Here is the method that allowed me to easily make such a ligature: tie one end of a thread (ligature silk) to a fine needle with a knot; then pass the other end through the eye of the needle to form a loop that can be tightened by moving the knot along the needle and allowing the thread to slide through the eye: choose the size of the eye in such a way that the friction of the thread is enough to prevent untimely slipping]. The needle is then inserted into a cork, which can be oriented as needed. An ocular micrometer allows the identification and measurement of any small displacements of the points under consideration.

In Araneus , when fixed by the thorax and observed from the side, one can see the posterior end of the opisthosoma making small swaying movements in the sagittal plane, with a frequency of about 130 per minute and variable amplitude, around 1/100 of a millimeter. The entire opisthosoma participates in this movement, oscillating around the pedicel. If the opisthosoma is fixed in a way that the prosoma remains suspended, then the prosoma exhibits an equivalent swaying motion. On the other hand, the pedipalps, as well as any free leg, oscillate at the same rate: the angles formed by the distal segments open and close alternately, so that the tarsi appear to beat in time with the movement, which they amplify due to their length.

One immediately thinks, as the cause of these phenomena, of sudden variations in blood pressure resulting from heartbeats; however, this organ is invisible because it is hidden by opaque integuments and tissues. The translucency of certain regions of the legs allows us to verify this hypothesis: the centrifugal current that can be observed along the axis of the appendages is too rapid for analysis, but in the superficial regions where the current is centripetal, there are branches of this current between muscle fibers, where blood cells progress distinctly at a jerky pace corresponding to the mentioned rhythm.

If one decreases the blood pressure by allowing blood to flow out through a sectioned leg, simultaneously with other phenomena we will discuss later (collapse of the dorsal region of the abdomen, relaxation of the joint membranes of the legs...), the rhythm of all these oscillations slows down equally and simultaneously. And if the bleeding is sufficient, the swaying movements cease, while movements in the dorsal midregion persist, which still allow, as we will understand later, counting the heartbeats.

The study of the structure of the heart could explain these observations [The best recent work dealing with the circulatory system of spiders, I believe, is the relatively little-known paper by Mr. Causard, Recherches sur l'appareil circulatoire des Arane̕ides (Bulletin scientifique de la France et de la Belgique, t. XXIX, 1896); but the figures, having undergone excessive reduction, are often difficult to read. The paper by Schimkevitsch, Etude sur l'anatomie de l'Epe̕ire (Annales des Sciences naturelles, se̕r. 6, t. 17, 1884), often cited, is almost unusable; I could not obtain the papers by A. Schneider, published in a collection that is currently unavailable to me. Furthermore, since physiological research requires a more precise anatomy of the heart than that shown by recent authors, who were mainly focused, curiously enough, on fine arterial ramifications, I had to devote a very long time to the revision of minute anatomical details. I only publish what is strictly necessary for my presentation and will be content with figures and legends: the details they contain were observed in serial sections and dissections, the preparation of which is delicate.].

Let us recall that the heart is a voluminous arc-shaped tube ( Figure 1 ), whose walls consist mainly of striated muscle fibers arranged in a circular pattern. It has three pairs of orifices, the ostia, in the form of slits, arranged as indicated in the figure, and bounded by two muscular lips protruding into the organ's cavity, serving as valves.

It continues with an anterior aorta, which is lodged in the hollow of a thick chitinous piece, the tergal part of the pedicel, whose rigidity protects it against deformation that could result from flexion of the opisthosoma. There, this aorta presents a sigmoid valve previously observed, according to Causard, by Aime̕Schneider. It has well-studied branches in the cephalothorax, some of which I represent schematically, partially based on Mr. Causard's description.

The heart ends posteriorly with a caudal artery, and three pairs of relatively small lateral arteries in the opisthosoma, as indicated in Figure 1 .

The heart is housed in a pericardial cavity, bounded by a thin conjunctive membrane applied to the surrounding lobes of the digestive gland. It is suspended there by a whole system of fine conjunctive tracts that extend to the conjunctive layer that lines the integument. Among these ligaments ( Figure 3), we can distinguish:

a. Epicardial ligaments, numbering ten pairs, spaced as shown in Figure 1 , plus an eleventh single bundle;

b. On the lateral eminences of the heart, i.e., on the three enlargements corresponding to the ostia, and on two very slight intermediate protrusions, pairs of groups of bundles are inserted, which Causard refers to as exocardial ligaments. In the most complex case, i.e., at the level of the ostia, each group includes: 1. the pteropyles, inserted on the lips and having a substantially vertical course; 2. the commissural bundle (wing muscles of various authors), inserted at the lower corner of the ostium and having a lateralposterior course. These two groups of fibers are also observed in the anterior ostia, but there they lie entirely in the pulmonary vein (this will be discussed later). At the level of the intermediate eminences and at the posterior end, the two groups can still exist separately (anterior intermediate eminences) or merge into one.

c. On the lower face of the heart, hypocardiac ligaments are inserted, five pairs: one very small pair very close to the anterior end of the heart, the second above the pulmonary veins, and the remaining three between the lateral arteries. They attach to the lower part: the first two pairs to the walls of the pulmonary sinuses, and the last three to the ventral muscular chain.

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With the help of this anatomical information, we can understand the origin of the swaying of the abdomen, particularly its collapse during systole [contraction of the heart]. I see two causes for this: first the straightening of the aorta due to the increase in internal pressure, and second the simultaneous increase in the curvature of the pericardial sinus, as contraction of the heart naturally leads to a drop in pressure in the pericardial cavity, and because of its specific shape, it must undergo a deformation opposite to that of the origin of the aorta, the effect of which adds to that of the first factor.

Deformation of the heart is transmitted to the body's walls through the suspensory ligaments; and it is evident to me that these ligaments, originating from the organization of a more irregular conjunctive network, have precisely the directions that correspond to the directions of the local tractions exerted by the walls of the heart and pericardial sinus on the walls of the opisthosoma. For this reason, I mentioned earlier, with some details, the distribution of these ligaments and carefully indicated their appearance in the figures.

However, the noticeable movements of the abdomen are more complex than the swaying we have just studied.

1. A series of setae corresponding to the anterior dorsal mid-region do not move parallel to themselves, as would be expected with a simple participation in the general rotation around the pedicel: they exhibit various tilting movements, which rhythmically change their respective orientations and show a particular deformation of their basal field, albeit very slight [This method of investigation, highly sensitive, is an adaptation on a microscopic scale of the procedure used by Plateau at times; a procedure that involved gluing long strips of stiff paper to certain points on the bodies of his insects to amplify changes in curvature on the surface being studied.]: a sagging that accompanies the cardiac systole. On the other hand, it can be observed, by immobilizing the end of the opisthosoma, that this deformation is independent of the general swaying of the abdomen. It is easily explained by the greater traction of the dorsal ligaments inserted on the portion of the heart that contracts the most during systole.

2. Just in front of the abdomen, in the vertical portion that overhangs the pedicel, there is an area that is projected forward, by about 1 / 125 of a millimeter, with each beat of the cardiac rhythm. It indicates that the beginning of systole causes a momentary expansion in the underlying portion of the vessel due to the projection of the blood contained in the regions situated further backward.

3. The pulmonary region of the integument sags during systole. We will study this phenomenon in Pholcus phalangioides .

Figures

Figure 1. The book lungs of Pholcus phalangioides (1) and Lyssomanes viridis (2-4) can be observed through the transparent cuticle of the operculum (arrows). However, observation of the movement of the air sacs within these lungs requires an aligned view parallel to the plane of these sacs at a relatively high (~144 X) magnification (Hill 1977). 3-4, The rapidly pulsating (or oscillating) flow of hemolymph through the pulmonary veins (P), heart (H), and anterior aorta (A) is easy to observe. Other abbreviations: L, book lung; dg, digestive gland, visible through the transparent cuticle.