The Nymph (Pl. 33, Fig. 6) of Stenoperla is of the campodeiform type, about 2.4 cm. in length just prior to the visible appearance of the wing-buds. The entire body is flattened dorso-ventrally, varying in colour from bright green to a reddish brown, depending on the dominant colour of the habitat. The ventral surface varies from very light green to whitish green; whilst the intersegmentalia vary from this latter colour to pink.
The Head (Pl. 33, Figs. 7 and 8) is flattened dorso-ventrally, slightly convex above, about as long as broad, with a distinct Y-shaped epicranial suture, the arms of which extend to the base at the antennae. The compound eyes are large, with numerous ommatidia, reddish brown, being on the lateral aspect of the epicranium. There are three ocelli situated between the eyes. On either side of the median ocellus, and anteriorly on the frons, are two small but distinct depressions, the invaginations of the dorsal arms of the tentorium, and two larger ones whose function is unknown, lie in the front. The frons is a large plate twice as broad as long, separated from the elypeus by a distinct clypeal-suture. It bears the pits mentioned above and the ocelli; the vertex is not differentiated as a separate sclerite. The gena extends laterally and ventrally to form the lateral and latero-ventral aspect of the head, and meets the mentum and gula on the ventral surface. It bears on its postero-lateral border a small tuft of fine setae (P. 33, Figs. 7 and 8). The clypeus is three times as broad as long, undivided, and has two small depressions lying in front of those on the frons. The breadth of the labrum is four times the length, and is separated from the clypeus by a distinct clypeo-labral suture. The antennae are long, multi-articulate, filiform appendages in which the scape is the longest and the pedicle a little shorter than the former; there are numerous other small segments. The head articulates with the thorax by a very short intersegmental region.
The Tentorium (Pl. 32, Fig. 4) is made up of the normal elements. There is a body from which a pair of arms pass dorsally forwards—the anterior arms—to the lateral clypeo-frontal region, where the gena joins these two latter. They give rise to the condyle on which the jaws articulate, and thus support these powerful organs. Dorsally, and mid-way along the anterior arm, the dorsal arm is given off to the head, and is visible externally in the two pits on the frons mentioned above. Posteriorly the posterior arms pass back to the occiput. The body of the tentorium descends ventrally and joins the gena of either side. It supports the oesophagus, protects the suboesophageal ganglion, and serves for the insertion of muscles.
The Thorax (Pl. 33, Fig. 6).—The prothoracic tergum is oval twice as broad as long; the meso-thoracic similar in shape and half as wide again as long, whilst the meta-thoracic tergum is similar to the latter. All these tergites show the primitive undivided condition.
The Pleurites (Pl. 32, Fig. 5). (Snodgrass 1909.) There is an undivided anteriorly placed episternum and a slender posterior epimeron, the latter being separated from the former by the pleural suture. There is no pre-episternum. The remaining two sclerites are latero-sternites according to Imms (1925). The more posterior sclerite is sickle-shaped, and articulates with the episternum, epimeron, and the coxa—the trochantin; whilst the more anterior one forms a large sclerite lying between the episternum and the sternum and forms the “ante-coxal plate,” which is homologised with a similar structure found in the cockroach and quoted by Balfour-Browne (1932). That it is not pre-episternum is concluded from the fact that, were it such, it would have to slip downwards and backwards beneath the episternum to reach its present position, a condition which seems most improbable. It is not presternum, as will be seen from Pl. 35, Fig. 14, where it is shown that this sclerite is small, does not reach the lateral margins of the ventral surface, and, furthermore, lies anterior to the sclerite in question. On the above grounds it is concluded that it is a latero-sternite and is homologous with the ante-coxal plate of the cockroach.
The Sternites (Pl. 35, Fig. 14).—The presternum is composed of a single, large, oval plate formed by the fusion of sternum and sternellum; evidence for this is seen in that posteriorly and laterally, a pair of furcae arise, thus identifying the sternellum (Imms, terminology); the anterior portion of the plate must then be composed of the verasternite of Martin (1916) and the sternum of Imms (1925). Now, according to Martin, the thorax is made up of five different subdivisions, and although direct evidence is at present lacking, the presternum and post-sternellum are either lost or have become fused with the sternum and sternellum. The furcae mentioned above are united at their distal ends with the pleural arms, thus forming a very strong internal connection between pleuron and sternum. Externally, two pits (Pl. 35, Fig. 15) lying between the legs of each segment are visible; these are the invaginations which give rise to the fureae. They correspond to the pits observed by Newport in Pteronarcys (1851).
Posterior to the presternum is a very small spinasternite, largely made up of the spina and serving for muscle insertion. The spina in Stenoperla is homologised with the thoracic pits of the cockroach (Miall and Denny 1886), since it is situated in a similar position and resembles them in appearance. It is thus the homologue of the spina in the Orthoptera.
The Mesosternum has a small oval sclerite, the presternum, situated just posterior to the spinasternite of the prothorax. Behind this lies a large plate composed of sternum, sternellum, and possibly post-sternellum; from the lateral aspect of which arise a pair of furcae similar in every respect to the preceding. Posterior to this sclerite there is another small spinasternite with its spina.
The Metasternum is made up of two plates, the anterior of which forms the presternum and is oval in shape. The posterior plate is composed of the same sclerites as that in the mesothorax,
but again, as in the case of the two preceding regions, evidence is lacking as to whether the post-sternellum is fused with the plate or is absent. Two furcae arise in a similar position and resemble the preceding ones in every respect. There is no spinasternite present, but if Martin's statement that the spinasternite probably belongs to the segment in front is correct, then this is to be expected. Again, as would be expected, and supporting this view, no spinasternite is found in the anterior region of the prothorax.
The Labrum (Pl. 33, Fig 7, and Pl. 34, Fig. 9) is an undivided sclerite four times as broad as long, having on its anterior median border a broad, shallow indentation. Its whole outer edge is provided with numerous macrotrichia.
The Epipharynx is formed of a membrane closely adherent to the labrum on its ventral surface, and is densely clothed with macro-and microtrichia. The macrotrichia are disposed on the lateral regions, and are probably of a tactile nature. There is no sign of differentiation.
The Mandibles (Pl. 33, Fig. 8, and Pl. 34, Fig. 10) are very strong and undivided, with a number of small, sharp, heavily chitinised teeth; no molar region; they are adapted for seizing, cutting, and tearing their prey to pieces. They articulate with the head by a condyle and ginglymus. On their inner aspect a large chitinous plate, the adductor apodeme, is attached to the powerful adductor muscles and represents the chitinised tendon. This plate lies ventral to the anterior and dorsal arms of the tentorium.
The Maxilla (Pl. 33, Fig. 8, and Pl. 34, Fig. 11) has a cardo divided into two, a proximal region or basicardo, and a distal region or disticardo. The disticardo is attached to a long, slender, undivided eustipes, which on its outer lateral aspect gives off a five segmented palp. No micromere as described by Crampton (1923) in Eusthenia was observed. The lacinia is well developed, and bears at its distal end two heavily chitinised teeth; the galea is divided into two, a proximal smaller region or basigalea, and a longer distal region or distigalea, the latter of which is not chitinised, but is fleshy.
The Labium (Pl. 33, Fig. 8, and Pl. 34, Fig. 12).—The mentum is a large, well-developed plate two-thirds of the length of the labium, and forms the floor of the head except for a very small posterior gular region. The prementum is partially divided, giving rise on either side to a palpiger, from which a three-segmented palp arises. The ligula is well-developed, having both glossae and paraglossae present, of which the latter are the larger.
The Hypopharynx (Pl. 34, Fig. 13) lies on the upper surface of the labium and consists of a single undivided plate, whose median anterior margin is produced forwards to form an obtuse angle. From the sides of this plate arise the bilobed superlinguae, the anterior lobe of which bears large macrotrichia probably of a tactile nature. From the median posterior margin a process continues backwards,
bifurcating into two arms which support the salivary duct of each side; on to this median prolongation the united ducts run forwards to open on the hypopharynx on the side apposed to the labium.
The Legs consist of seven segments as follows: A small undivided coxa articulates with the trochantin, and articulates on its distal border with a small undivided trochanter. This is followed by a long stout femur and tibia, the latter bearing a terminal, heavily chitinised spur; and, finally, by a three-segmented tarsus of which the distitarsus is the longest, bearing a pair of very strong terminal ungues. The other two segments are short and equal in size; there is no empodium, nor are there any pulvilli. In life the tarsus is brown, whilst that of the other segments is green.
The femur and tibia bear on their postero-lateral borders long natatory hairs, used by the nymph to assist the lateral undulations of the body during swimming.
The basi- and medio-tarsal segments bear on their ventral surface two rows of very stout bristles (Pl. 35, Fig. 16), which are situated around the margins of two depressions, one on each segment, and these, together with the terminal ungues and tibial spur, form a very efficient crampon by means of which the nymph is enabled to hold on against very swift currents. That the structure is purely an aquatic adaptation is indicated by the fact that it is lost in the imago and is replaced by two pads.
The Abdomen (Pl. 33, Fig. 6) is ten-segmented, with the first segment reduced in size. On either side of the mid-dorsal line, one pair to each segment, are two rows of small round areas forming a constant pattern and lighter than the general body-colour. On the last segment there are three, the odd one being situated posteriorly on the midventral line. Five pairs of white, segmented, tracheal gills are present on the first five abdominal segments. The abdomen bears on the last segment a pair of filiform, multi-articulate cerci which are about one-third of the total body length, green on the proximal and brown on the distal segments.
Alimentary System (Pl. 40, Fig. 33).—The alimentary canal can be divided into the following regions:—
1. Foregut (Stomadoeum)
3. Hindgut (Proctodaeum)
The Mouth is bounded above by the labrum, laterally by the mandibles and maxillae, and ventrally by the hypopharynx and labium. This leads into a muscular—
Oesophagus, which runs the entire length of the head ventral to the brain and dorsal to the body of the tentorium, behind which the oesophagus bends downwards and passes along the floor of the head, giving off radial muscles to the roof, sides, and floor as it goes. In the region of the cervicum the oesophagus constricts and passes backwards into the prothorax, where it gradually expands out into a very thin-walled sac, the—
Crop, which extends back to the middle of the metathoracic segment. When empty or only partially filled with food the walls are thrown into longitudinal folds. Posteriorly it narrows and passes into the—
Gizzard, which is very difficult to distinguish externally except in those cases where the foregut is filled with dark food material, when the greater thickness of its walls render it visible.
The Mesenteron follows the gizzard and joins the latter at the junction between the metathorax and first abdominal segment. It is a long straight tube extending back to the sixth abdominal segment, narrowing towards its central region, but widening again posteriorly. At its anterior end two very short, forwardly directed caeca are given off laterally. Its posterior half is pigmented with cylindrical pigment granules.
The Hind-gut extends from the sixth abdominal segment backwards, and, at its junction with the mesenteron, gives off a large number of Malpighian tubules (50 to 60), which run forwards to find attachment on the posterior region of the gizzard. The anterior portion of the hindgut dilates to form the ileum, which, in turn, constricts posteriorly to pass into another dilation, the colon-rectum, extending to the anus, which discharges on the tenth segment between the cerci. The posterior region of the rectum, as in the oesophagus, gives off radial muscles and constricts to form a sphincter. Glandular and Excretory Structures of Alimentary Canal.
The Salivary Glands (Pl. 40, Fig. 33) consist of a pair of racemose, whitish bodies, deeply embedded in and closely resembling the fat-body. There are two pairs, one on either side of the prothorax. The more posterior gland lies dorso-laterally on the crop, to which it is closely adherent, the more anterior is situated near the cervicum and is deeply embedded in the fat-body, from which it is difficult to separate. Each gland consists of a number of acini opening into a very slender salivary duct, the more posterior duct passing forwards to be met by a similar duct from the anterior gland. The common duct passes forward mid-laterally along the oesophagus till near the mouth, where it descends and meets its fellow from the opposite side mid-ventrally. The common duct so formed runs forward on the horn of the hypopharynx to open on this latter
structure on the side apposed to the labium (Pl. 34, Fig. 13). There is no salivary reservoir, and the main ducts are provided with taenidia.
The Malpighian Tubules (Pl. 40, Fig. 33).—These vary from 50 to 60 in number, are long, thin tubules about 1 cm. long, and are arranged in a double row around the mesenteron, each opening into the latter by means of a separate opening situated on the apex of a small papilla. (Histological evidence for this will be presented in a later paper.) For about two-thirds of their distal length the lumen is clear and well-defined, but in the proximal third becomes reduced in size by the approximation of the cells and the accumulation of solid urates. The distal portion is lined internally by a darkened membrane, the striated border (Wigglesworth), which, according to Wigglesworth (1931), cannot be separated into its constituent striae. This border becomes more evident in the proximal region, and its constituent elements become discernible. In a freshly killed specimen each tubule exhibits a slow serpentine movement. All are well provided with trachea, the tracheoles of which run along their length. The distal region of each tubule is produced into a delicate terminal filament by means of which each is attached to the gizzard; thus far no reference to this has been found, and Wigglesworth does not appear to have observed anything of this nature in the Reduviid bug. A similar condition has been observed in another stonefly nymph.
The Fat-body consists of two pairs of organs which attain their maximum size in the abdomen, but extend forwards into the thorax and head. The dorsal pair lie dorso-laterally and are situated in the pericardium; they constitute the pericardial fat-body. The ventral pair, the adipose fat-body, occupy a ventro-lateral position, but lie beneath the gut and gonads.
The Nervous System.
The nervous system of Stenoperla is divided into the following parts:—
1. The brain situated in the head above the oesophagus.
2. The sympathetic nervous system.
3. The sub-oesophageal ganglion.
4. The ventral nerve cord.
The Brain (Pl. 36, Figs. 20 and 21, Pl. 40, Fig. 34) is a large mass of nervous tissue situated in the head above the oesophagus, and extends over the greater part of this region between the compound eyes. It is convex above and concave beneath, with the anterior and dorsal arms of the tentorium passing between the antennary and optic nerves, and is wider than long. Three parts are distinguishable:—
1. The Proterocerebrum.
2. The Deuterocerebrum.
3. The Triterocerebrum.
To understand the relationship of these three, the one to the other, it is necessary to investigate the condition of the segments which originally gave rise to them. Primitively, the mouth was
ventral in position, but has moved forwards to take up a position at the extreme anterior portion of the body, with the result that the clypeus and labrum have been pushed up and lie dorsal to it. Similarly, the deutero- and triterocerebrum have moved forwards, the former upwards over the oesophagus and through 90°, taking the triterocerebrum with it; the proterocerebrum lies behind these and has rotated through 90°. The triterocerebrum now lies on either side of the oesophagus and not above it, hence the deuterocerebrum has come to overlie the former.
The Proterocerebrum consists of a pair of lateral lobes united along their middle line, and forms the greatest and posterior region of the brain. Each lobe gives off a nerve on either side to the lateral ocellus of its side, and anteriorly between the junction of the proteroand deuterocerebrum the nerve to the median ocellus is given off. Attached to the proterocerebrum on either side is a large optic lobe, wider than it is long, directed laterally. It gives rise to the optic nerve. Posteriorly and ventrally the proterocerebrum gives off the hinder region of the sympathetic nervous system, but it is possible that this may retain an internal connection with the triterocerebrum.
The Deuterocerebrum consists of two well-developed lobes, one on either side directed latero-anteriorly, and connected to one another by a piece of nervous tissue forming the dorsal lobe of the brain. Each lobe gives off a double antennary nerve to the antenna of its side, the more anterior nerve being the larger.
The Triterocerebrum lies ventrally to the deuterocerebrum on either side of the oesophagus. Each trito-cerebral lobe is connected with its fellow by a post-oesophageal commissure, which descends from each lobe and encompasses the oesophagus. It also gives rise to the two large para-oesophageal commissures which run downwards and backwards to the suboesophageal ganglion, and to the labro-frontal nerves which form the root of the frontal ganglion.
The Sympathetic Nervous System.
The sympathetic nervous system (Pl. 36, Figs. 20 and 21, Pl. 40, Fig. 34) resembles that of the saltatorial Orthoptera, in that there is a paired stomatogastric nerve, each with its own stomachic ganglion. A small triangular frontal ganglion lies in front of the brain, giving off anteriorly a nerve to the clypeus. Laterally, two nerves run lateroanteriorly and arch backwards to find connection with the labrofrontal nerves of the triterocerebrum. Each of these nerves gives off another pair which run forwards to innervate the mouth parts. Posteriorly the frontal ganglion gives off the recurrent nerve, which runs backwards beneath the brain to join a mass of nervous tissue representing the fused oesophageal and hypocerebral ganglia. It is probable that the corpora allata are included in this fused mass, as they are not represented elsewhere. A pair of bodies, at first thought to be these were afterwards revealed to be stomachic ganglia. From the antero-lateral aspect this mass gives off two connectives to the brain; postero-laterally on either side there arises a stomatogastric nerve, which descends to a medio-lateral position on the oesophagus and runs backwards to the cervicum, where it gives off a small whitish
body, the stomachic ganglion. Thence it runs backwards parallel to the salivary duct, finally arborising on the anterior caeca of the mid-gut (Pl. 40, Fig. 33). The ventral system has not been traced, but paired transverse nerves have been observed associated with the ganglia of the ventral nervous system.
The Ventral Nervous System (Pl. 37, Fig. 22).
The ventral nervous system consists of twelve ganglia; a sub-oesophageal, three thoracic, and eight abdominal, all connected by a double nerve cord. The abdominal ganglia all clearly exhibit a double nature.
The Suboesophageal Ganglion is a large, well-developed ganglion lying beneath the oesophagus under the proterocerebrum. It lies on the mentum and is protected dorsally by the body of the tentorium; anteriorly, it gives off nerves to the mandibles, maxillae, and labium, and is connected to the triterocerebrum by the para-oesophageal connectives. It narrows posteriorly and gives off the paired ventral nerve cord.
The Thoracic Ganglia are three in number, each lying in a median ventral position in its segment; all are equal in size. Each gives off a pair of large crural nerves to the legs, and smaller ones to the muscles. They are connected the one to the other and to the suboesophageal ganglion by the paired nerve cord.
The Abdominal Ganglia are clearly divided into right and left halves, and, as before, are connected by a double nerve cord. The first abdominal ganglion lies in the metathorax immediately behind the metathoracic ganglion, and has migrated forwards, but is not fused with the latter. A similar condition is found in dragonflies in some of which this ganglion has actually fused with the thoracic. It gives off a pair of nerves backward to innervate the first abdominal segment.
The remaining seven ganglia lie in segments two to seven. From the anterior portion of the second ganglion a pair of nerves pass outwards and innervate the muscles, etc., of that segment; segment three is innervated by a pair of nerves arising from the connectives anterior to its ganglion, whilst posteriorly to this latter another pair of nerves arise to innervate segment four. Postero-laterally from the fourth ganglion a pair of nerves pass out and innervate segment five, whilst the fifth ganglion innervates segment six by a pair of nerves exhibiting double roots; ganglion six innervates the posterior region of segment six and the anterior of segment seven; the seventh ganglion lies in the intersegmental region between segments six and seven, and gives off a pair of long nerves which pass back to segment eight, innervating the major portion of segment seven as they go. The eighth and last ganglion is larger than the preceding ganglia, and gives off posteriorly a pair of long nerves which pass back into the cerci, innervating segments nine and ten as they pass.
The Reproductive System.—This resembles that of the adult described later; it appears very early in nymphal life.
The Circulatory System.
The heart lies dorsally in the abdomen in segments one to nine. It is a long, delicate tube consisting of ten chambers separated from one another by constrictions, which correspond in position to the intersegmental regions. Anteriorly it continues forwards as a fine aorta to the head, ending beneath the brain. Laterally each chamber opens into the pericardial sinus by paired ostia, and each gives off on its lateroventral aspect two pairs of alary muscles which are attached on either side to the anterior and posterior regions of their segment.
The heart is enclosed in a space, the pericardial sinus, bounded above by the tergum and beneath by the diaphragm; lying in this sinus and on either side of the heart is the pericardial fat body. So far the muscles have not been seen to display striations.
The Respiratory System.
The tracheae (Pl. 37, Fig. 23) consists of a pair of longitudinal, lateral trunks extending backwards from the prothorax. Anteriorly each trunk bifurcates and supplies the head. There are five tracheal gills on either side of the first five abdominal segments.
The Prothorax and Head.—In the region of the prothorax the longitudinal trunk bifurcates into a dorsal and ventral trachea, the latter of which runs practically straight forwards, sending off a large branch to the legs and smaller branches to the cervicum and ventral muscles of the head, bifurcating at its anterior extremity to send one branch to the ventral mouth parts, the other up to the antenna. The dorsal branch runs straight forwards to the cervicum, sending off smaller branches to the muscles in this region, it then curves laterally following the contour of the head, giving off numerous trachea to the muscles, eyes, etc. Anteriorly, beyond the eyes, it curves inwards, giving off two small branches to the antenna, and finally arborises on the clypeus. No commissural connection with its fellow on the opposite side has been traced, any such connection taking place through the tracheoles (cf. adult).
The Meso- and Meta-thorax.—In this region the trunk is single and curves inwards towards the middle of each segment and outwards towards the intersegmental portion. The anterior region of the trunk in each segment gives off a branch laterally to the legs, this latter joining a similar branch from the posterior region. In this way the typical Y-shaped tracheae to the legs as figured by Comstock (1918) is formed. The posterior, metathoracic leg branch receives a trachea from the first abdominal gill.
The Abdomen.—Between segments two to six the main longitudinal trunk curves inwards towards the middle and outwards towards the intersegmental region. The tracheae of abdominal segments two to five are all similar, so a typical segment will be described. In segment two, however, the first branch is a large one running inwards to the gonads; this is not present in the other four. Externally, in the anterior region of each segment, the trunk receives a trachea from the gill; from this branch three others are given off, the largest of which passes inwards and arborises on the gut, joining
its fellow from the other side by tracheoles only (cf. adult); no true commissures are present. The other two branches are smaller and arborise on the muscles and fat body.
From the posterior region of segment five the trunk gives off a large branch internally to the malpighian tubules, and externally just in front of this a small branch to the fat body. In segments six and seven the trunk is much straighter and gives off ventrally a large branch which curves round internally to arborise on the posterior region of the midgut; numerous small branches pass from the trunk to the fat-body and muscles. Between segments seven and eight the main trunk turns outwards, and at this point gives off a large branch internally to supply the whole of the hind-gut. After passing outwards for a short distance, the trunk turns and runs straight backwards to segment ten, bifurcates, and finally passes into the cercus. As it passes, it sends off branches to the fat-body and muscles in segments eight, nine, and ten.
Here and there along the main lateral trunk and on the leg trunks there are white opaque collars.