THE MOTOR MECHANISM OF THE INSECT LEG

Transcription

1 22O THE MOTOR MECHANISM OF THE INSECT LEG BY J. W. S. PRINGLE From the Department of Zoology, Cambridge University (Received 28 December 1938) (With Six Text-figures) INTRODUCTION IN view of the number of papers that have appeared in recent years on* the neuromuscular systems of various groups of animals, it is somewhat surprising that no attempt has been made to clarify the position in regard to the insects. There can be no doubt that a particular interest attaches to this group from the standpoint of comparative physiology. As the most highly developed members of the Arthropod phylum they represent the end of an important evolutionary line; as successful colonists of the land they have been faced with some of the same difficulties that confronted the higher Vertebrates, and in many cases have found different solutions. Nowhere is this more obvious than in the problems of locomotion. On the sensory side we find the proprioceptive system of insects highly developed and differing radically from that of the Vertebrate (Pringle, 1938 a, b), and the present paper will bring out further differences in the motor mechanism. Perhaps we should regard as the most interesting feature of the whole comparison the very fact which has served to mask the inherent differences: the extraordinary outward similarity of the locomotory mechanisms in the two groups; a similarity imposed on essentially different animals by the exigencies of a common environment. REVIEW A complete understanding of the mechanism of the insect leg as a motor organ cannot be reached without a knowledge of its detailed anatomy. Snodgrass has given the most complete description of the musculature, in the grasshopper Dissosteira, and the work is summarized in his textbook (Snodgrass, 1935). Owing to differences in the shape of the segments of the leg in different insects, it is not possible to give more than a rough functional classification of the muscles, but one can usually distinguish depressor and levator sets, serving respectively to raise and lower the animal relative to the ground, and promotor and remotor sets pulling the leg forward and backward. Nevertheless it is found that homologous muscles in related insects may sometimes have different functions (cp. the jumping muscle of the grasshopper, the extensor tibiae, which is normally a levator muscle).

2 The Motor Mechanism of the Insect Leg 221 The histology and physiology of the individual muscles and their motor nerve fibres make up the other side of the problem. Here we must consider also the wing muscles, which have been extensively studied. Histology Mangold (1905) described the histology of the motor nerve distribution in Decticus (thoracic and leg muscles) and Dytiscus (wing muscles). He found in each case a double innervation, the two fibres being distributed side by side throughout the muscle. Mangold relied for his preparations on the methylene blue vital staining method, but several more recent workers, using different silver methods for studying the nerve endings in the muscle, have not confirmed his results. Thus Sanchez (1913) figured a single nerve supplying several muscles in the head of Apis, and Marcu (1929) described the endings in Orthoptera, Coleoptera and Diptera. This last author was more concerned with the nature of the so-called "Doyere's cones" or end-plates than with the question of double innervation. He found that in Orthoptera the nerve fibres branched dichotomously and ended simply on the surface of the muscle fibres, but that in higher insects there was a definite region of fanshaped branching where each nerve twig ended on the muscle fibre; this formed the end-plate. Marcu's figures show only single nerve fibres supplying the muscle fibres, but there are marked longitudinal "fibrillations" which might be taken to indicate the presence of more than one fibre. Physiology Much of the physiological work on insect muscles has centred round the problem of the mechanical system of flight. The high frequencies found in the flight of some insects (up to 200 per sec. in Calliphora (Magnan, 1924)) have been hard to reconcile with the slow tonic contraction of the leg muscles of others. Owing to the difficulty of dissecting out a sufficient length of motor nerve, many workers have adopted the method of direct electrical stimulation of the muscle. Thus Solf (1931), using the extensor tibiae of Decticus, found that summation occurred and a smooth tetanus set in at 10 shocks per sec. Heidermanns (1931), working with a preparation of the large direct wing muscles of Aeschna, found evidence of fusion of contractions at about 20 per sec. As in the normal wing beat of Aeschna at 25 per sec. the contractions are quite distinct, he concluded that some other factor must be at work, and suggested (a) that the muscle must be working at or near the optimum loading at which its time of contraction and relaxation is a minimum, and further (b) that it does not contract fully each time, and so does not take so long to perform one complete cycle of contraction and relaxation. With half contraction at optimum loading the necessary frequency can be attained, and in the intact insect the contraction of the antagonistic muscle will also aid relaxation. Kraemer (1932) and Cremer (1935) produce evidence for a high value of optimum loading for Dytiscus leg and Aeschna wing muscles respectively. With directly applied single shocks both muscles show a maximum contraction rate at one particular

3 222 J. W. S. PlUNGLE loading, which is 40 times the muscle weight in Dytiscus extensor trochanteris, and 550 times in Aesckna flight muscle. With faradic stimulation the values are higher still, but are probably approached in the normal animal owing to the very high leverage factor produced by the method of articulation of the wings. Several workers have commented on the relation between intensity of direct stimulus and amplitude of response. Heidermanns (1931) found a direct proportionality. Solf (1931), however, states that in the normal unfatigued extensor tibiae of Decticus (the jumping muscle) the thresholds of all the muscle fibres are the same, so that the whole muscle behaves in an all-or-none manner. Kraemer (1932), studying the extensor trochanteris of Dytiscus, found four distinct steps in the height of contraction with increasing intensity of stimulus. In many of these cases it is not clear that care was taken to avoid stimulating the intramuscular endings of the nerve, and most of the apparently anomalous results can be explained on this basis. Only one attempt has been made at indirect stimulation; Friedrich (1933) studied the response of the muscles of the isolated leg of Dixippus to electrical stimulation of the nerve at the leg base. From his records he claims that peripheral inhibition is observable at an intensity of stimulation below that necessary for excitation; and, further, that this can be detected also with direct stimulation over the muscle. The so-called inhibition was apparent as a slight further relaxation of the resting muscle, or as an increase in the rate of relaxation after a contraction. The effect as shown in his published records is very small. For completeness, mention should also be made of the experiments of Buddenbrock (1920), by which he claimed to have demonstrated the existence of a special " tonus " mechanism in insect muscle, able to maintain a contraction with a lower rate of energy utilization than that needed to initiate it. Buddenbrock measured the rate of oxygen uptake of Carausius in two different cataleptic states: (1) lying on its back with its legs folded against its body, and (2) cataleptic standing with legs extended. The rates agreed to within 1 % and he concluded that the muscles active in the standing position must be tonus muscles. The argument seems to be at fault in supposing that the leg muscles were relaxed in the first position; with the type of antagonistic musculature found in the insect leg, any position from complete flexion to complete extension can be attained with the muscles in a considerable state of tonic contraction. Evidence of a different sort for a double mechanism was found by Rijland (1932 a, b). In the electrical record from the leg muscles of a number of different Arthropods, including Dytiscus and Hydrophihis, Rijland detected two different amplitudes of electrical response associated respectively with active contraction and tonus. The smaller impulses associated with tonus (amplitude about 100fiV.) tended to continue rhythmically for long periods at frequencies of from 2 to 40 per sec, being affected by movement of the limb and other reflex stimuli; the larger impulses accompanying active contraction (amplitude about 25O^iV.) occurred in short high-frequency bursts, and were always followed by a declining series of smaller impulses. Rijland did not investigate further the nature of the two types of response; his observations have been repeated and confirmed in the present work.

4 The Motor Mechanism of the Insect Leg 223 MATERIAL AND METHODS The American cockroach, Periplaneta americana L., has been used in the experiments to be described, and a particular study has been made of the metathoracic segment and the third pair of legs. One muscle of this leg, the extensor tibiae, has been especially studied in order to determine the range of normal activity, and the motor system of the whole leg is considered in the later part of the paper. Electrical recording has been done with a 4-stage amplifier, with alternative direct or condenser coupling, and a Matthews (1928) oscillograph. Owing to the small size of the material, fine platinum wire electrodes have had to be used and placed in position on the nerve or muscle by means of a Peterfi micromanipulation apparatus. RESULTS The responses of a single muscle The extensor tibiae muscle, with which this section of the paper deals, is particularly suitable for investigation. It is attached at one end to the base of the femur and at the other to a short apodeme on the outside of the femoro-tibial joint; owing to the length of the femur in which it lies, electrical records from it can be obtained by inserting the electrodes through the chitin at opposite ends of the segment without any further dissection. It is innervated from one of the small nerve trunks (nerve 3 b, Fig. 1) and is the most distal muscle supplied by this nerve, so that section of the other nerves close to the ganglion eliminates all other electrical activity in the femur except for sensory impulses in nerve 5, which are small and can easily be distinguished on the record. In the intact preparation with nerve 3 b attached to the ganglion two types of electrical response are obtained; a more or less regular series of spikes of small amplitude (approx. ioo^tv.) at frequencies in the neighbourhood of 30 per sec, and short bursts of larger spikes (approx. zun.) at frequencies from 7? per sec..""_.,. v, -... «1. Fl 8- ' Diagram of the nerves upwards. Thefirsttype of spike is accompanied by a leaving the metathoracic gangtonic contraction; the short bursts of large spikes by Kon of Periplaneta. Nerve 1 does, r 1 e 1 / T^-M J / \ L \ not supply the leg; 2 and 7 are a brief and powerful tetanus (cp. Rijland (1932) above), tracheal trunks accompanied by For more exact determination of the nature of these a few fibres not part of the leg A., 1, j.. 1 motor systems. two responses in the muscle, the nerve was dissected out in the thorax, cut close to the ganglion, and stimulated electrically by short-duration condenser discharges through a Neon lamp (stimulator as used by Pantin (1934)). Fig. 2 shows the typical mechanical response at three different frequencies of stimulation of the nerve. In this preparation the

5 224 J. W. S. PRINGLE apodeme of the flexor tibiae was cut and the movement of the tibia resisted by a small coil spring. The leg was set up so that the image of the tibia was focused on the camera by the concave oscillograph mirror; by feeding a part of the condenser discharge into the last stage of the amplifier a simultaneous record was obtained of Fig. 2. Mechanical response of extensor tibiae muscle to stimulation of quick fibre at frequencies of 19, 55 and 120 stimuli per second. Isometric recording at i6 C. Time marker I/IO sec. stimulus and response, enabling the frequency to be accurately determined (in the reproduction of the records the small stimulus markings are lost). The records show that at low frequencies the muscle responds to each impulse with a discrete twitch; fusion of twitches starts at about 30 per sec, and a smooth tetanus results at frequencies in excess of 70 per sec. Except in occasional preparations, special treatment is necessary in order to reproduce the tonic contraction by indirect electrical stimulation. If the nerve is allowed to dry to the correct point and then remoistened, or is moved about to several positions on the electrodes, this type of contraction can sometimes be obtained. Fig. 3 shows the mechanical response of the intact isolated leg under these conditions. A smooth tonic contraction occurs, and both the rate of the contraction and its final intensity depend on the frequency of the excitation, over very wide limits. No response is, in fact, obtained at frequencies lower than about 30 per sec. in the fresh preparation. Under high-frequency stimulation it is possible to obtain from this preparation a tension comparable with that produced by the twitch mechanism under tetanus. It seems clear that two different nerve fibres are responsible for the two types of response. Normally the threshold of the fibre producing the twitch (which we will call the quick fibre) is lower than that of the fibre producing the tonus (the slow fibre), and thus a twitch is obtained from stimulation of the nerve. This explains the failure of previous workers to detect the slow fibre. Only by drying the nerve

6 Fig. 3. Mechanical response of extensor tibiae muscle to stimulation of slow fibre at frequencies of 43, 72, ioo, 150 and 340 stimuli per second. Free movement of tibia at 16 C. Time marker 1/10 sec. The beginning and end of stimulation (in each case 2 sec.) have been marked with arrows. After the last stimulation, the muscle took several seconds to relax, possibly owing to after-discharge from the nerve. Fig. 4. Electrical record from extensor tibiae muscle during stimulation of nerve. Above: slowfibre stimulation at 116 stimuli per second. Below: quick-fibre stimulation at 47 stimuli per second. The amplification is less in the lower record. Time marker 1/10 sec.; direct-coupled amplifier.

7 226 J. W. S. PRINGLE until the quick fibre ceases to conduct, or by a chance placing of the electrodes, is it possible to excite the slow fibre alone. But in both cases, once the threshold is reached, the independence of the response of the intensity of stimulation shows that only one nerve fibre is concerned. The electrical responses produced by the two fibres in the muscle are shown in Fig. 4. The slow-fibre impulses produce even at low frequencies a muscle spike of about 100fj.V. (recorded as explained above); at frequencies above about 50 per sec. facilitation is apparent, and with steady stimulation the height of the muscle spike increases to a maximum, the relative increase in height being dependent on the frequency and reaching some five times at 150 per sec. The quick-fibre impulses produce a much larger electrical response in the muscle (about 2/xV.) and the height of the spike is constant at all frequencies. No evidence has been obtained of the presence in this muscle or elsewhere in the leg of inhibitory fibres. No change in the intensity of either the mechanical or electrical response is to be observed at any intensity of stimulation of the two nerve fibres above the threshold, or by stimulation of any of the other nerves to the leg; nor is there any evidence of a further relaxation of the muscle in response to subthreshold stimulation (as claimed by Friedrich, 1933). Direct evidence against the presence of an inhibitory fibre is provided by electrical records from the intact preparation when there has been a sudden reflex cessation of the tonic contraction; such records show a sudden stoppage of the series of muscle spikes without any gradual reduction in their intensity such as might be produced by the arrival of inhibitory impulses. Discussion of the double innervation It is of considerable interest to enquire whether the two types of response described above are produced by the same muscle fibres, and if so how it is possible to obtain in one case a twitch and in the other a smooth contraction. On the first point the histological work of Mangold (1905) shows clearly that in some insect muscles, at any rate, there is a double innervation of the individual fibres. Further to this, direct microscopic examination of the muscle while responding to quick- and slow-fibre excitation fails to reveal any difference in the locus of contraction; sometimes, particularly with high frequencies at which the slow fibre is producing a considerable response in the muscle, it is possible to see quite clearly that the same fibres are contracting. If this is the case, and the small size of the electrical spike produced by the slow fibre at low frequencies is merely a measure of the small number of muscle fibres excited at each impulse, then it is possible to explain the various differences as follows: (1) Type of contraction. At frequencies below about 30 per sec. the number of muscle fibres responding to the impulses in the slow fibre is so small that the twitches are damped out by the rest of the muscle and no appreciable contraction results. With rising frequency, by the time facilitation has brought into play a significant number of muscle fibres, the frequency is such that mechanical fusion is occurring to produce a smooth contraction.

8 The Motor Mechanism of the Insect Leg 227 (2) Differences in the electrical response. The similar time relations of the spikes produced in the muscle by the slow- and quick-fibre impulses suggest that the difference is merely due to a change in the number of muscle fibres excited. There is no indication, for instance, of any slow depolarization produced by the slow-fibre impulses, such as might produce a contracture in the muscle (the upper record in Fig. 4 is monophasic and would show any slow depolarization as a gradual shift of the base line). It should, then, be possible to trace a parallel between the height of the facilitated spike and the tension produced, at all frequencies of slow-fibre excitation, and to compare them with those produced by the quick fibre. Simultaneous recording of electrical response and tension has not been possible, but at the higher frequencies this is certainly true in a general way. Fig. 5 shows that at a Fig. 5. Electrical record from extensor tibiae muscle, showing effect of superimposing a single quick-fibre impulse on a background of slow-fibre impulses at a frequency of 185 per second. Slow-fibre stimulation at cut end of nerve; quick-fibre stimulation by electrodes in coxa. Time marker 1/10 sec.; condenser-coupled amplifier. frequency of 185 per sec. the quick-fibre spike is about three times the height of the facilitated spike of the slow fibre; the highest frequency at which measurements of tension have been made from the same preparation with each type of excitation is 83 per sec, at which the ratio was 1:5. The magnitude of the electrical spike is much influenced by the position of the electrodes and by the short-circuiting effect of the rest of the tissue; but with due allowance for the difference in frequency and for the fact that different individuals are concerned, the ratios are certainly of the same order. It has not been possible to record the electrical effect of excitation in a single muscle fibre. On the basis of the interpretation given above, the response of the individual fibre should be the same, whichever nerve was responsible for exciting it. A further point of similarity between the two types of response is seen in the effect of fatigue. As the preparation ages, the height of the spike produced by the slow-fibre impulse gets less and less, and the amount of facilitation at any given frequency becomes reduced. At the same time the quick-fibre response loses its allor-none character and also shows facilitation; in very fatigued muscles the response produced by the quick fibre comes to resemble that produced normally by the slow fibre; it is significant that under these conditions a smooth contraction occurs, similar to the normal slow-fibre contraction, and resembling it in being dependent on the frequency of stimulation. (3) The nature of the facilitation process. Microscopic examination of the muscle showing a tonic contraction in response to a low frequency of slow-fibre impulses

9 228 J. W. S. PRINGLE indicates that the contraction is not all being produced in a few of the fibres; all the fibres must be responding in turn, a certain number being excited by each impulse, but not always the same ones. For this to occur some "remainder" must be accumulated at each impulse. The double innervation of the cockroach muscle makes it possible to test whether this accumulation is of the nature of a summation of stimuli by the muscle fibre, or an accumulation in the nerve or neuro-muscular junction. Fig. 5 shows the effect, on the electrical response of the muscle, of superimposing a single impulse in the quick fibre on a high-frequency background of impulses in the slow fibre. At this frequency of slow-fibre excitation (185 per sec.) facilitation occurs at the start of stimulation to an extent of about six times and takes 1/10 second to develop its full value; the record shows that the excitation of all or nearly all of the muscle fibres by the quick-fibre impulse does not reduce the extent of the facilitation appreciably, and certainly does not abolish it altogether as would be expected if facilitation was in the nature of a summation of stimuli by the muscle fibres. The accumulation, whatever it is, must therefore either be in the nerve, possibly in some measure of negative retention, or in the neuro-muscular junction, as a chemical effect. Comparison between insects and Crustacea The neuro-muscular system of the Crustacean appendage has been studied by a number of workers, and Harreveld and Wiersma (1937) describe what is as yet the most generalized type. In the claw of Cambarus studied by them there is a triple innervation, including a quick, a slow, and an inhibitory fibre. The mechanical response of the muscle to stimulation of either of the excitatory fibres is a smooth contraction, different in rate for the two fibres; the inhibitory fibre impulses reduce the intensity of either contraction. The electrical records from the muscle indicate that the slow-fibre impulses show facilitation, the first few impulses being indeed completely ineffective; the quick fibre produces a maximal spike at each impulse. The parallel with the insect muscle is very close, and apart from the absence of the inhibitory fibre in insects the differences can be reduced to two factors. In the cockroach the slow fibre excites a few muscle fibres even at the first impulse; and mechanical fusion of twitches does not occur until a frequency of about 30 per sec. (at 16 C.) is reached. In the crayfish the first few slow-fibre impulses are ineffective ; and smooth summation of contraction occurs at all frequencies. The variety of types of innervation found by Harreveld and Wiersma (1937) in different muscles of the Crustacean appendage suggest the possibility that a different arrangement to the one described may be found elsewhere in the insects. The possibility of the occurrence even of inhibitory fibres cannot therefore be ruled out a priori. It is, however, possible that there is less need for such aids to rapid relaxation in an animal where the speed of the mechanical system has been so increased ; a general survey of the innervation of the other muscles of the cockroach leg fails to reveal any significant differences from the system in the extensor tibiae.

10 The Motor Mechanism of the Insect Leg 229 The muscular system of the leg Having considered in detail the behaviour of a single muscle, we may now pass on to a more general study of the whole leg. In order to provide a complete picture tg.prm.cx. -A pl.dpr.cx. pl.rm.cx.-^l st.prm.cx. ext.tr. 3 ext.tr.vn. ----flex.tr. ---flex.tr. ext.tr. ds. Fig. 6. Muscles of the coxa and trochanter of Illrd left leg of Periplaneta, showing diagrammatically the anatomical details of the innervation. 3, 4, 5, 6, nerves from metathoracic ganglion (cp. Fig. 5); wing, point of articulation of wing; ext.tr. 4, pleural portion of extensor trochanteris; pl.dpr.cx., pleural depressor coxae; st.prm.cx., sternal promoter coxae; ext.tr. i, 2, 3, coxal portions of extensor trochanteris; flex.tr., flex".tr., flexor trochanteris; ext'.tr.ds., ext'.tr.vn., dorsal and ventral portions of accessory extensor trochanteris; pl.rm.cx., pleural remoter coxae; tg.rm.cx., tg.prm.cx., tergal remotors and promoters of coxa; pi., pleuron; ex., coxa; tr., trochanter;fm., femur. of the action of the leg as a motor organ, an attempt was made to map the distribution of all the motor fibres supplying the more distal muscles of the appendage. Although this has not been completed, enough has been done to show the nature of

11 230 J. W. S. PRINGLE the system and the formation of a complete list would not be a matter of very great difficulty. The method chiefly used in tracing the nervefibreswas essentially physiological, based on the use of nicotine. This drug, as described by Langley and Dickinson (1889),fi *excites and then paralyses synapses. In the insect ganglion the excitation takes the form of a train of impulses in the nerve, rising to a maximum frequency of from 100 to 800 per sec. and then rapidly declining. Different nerve fibres are excited in turn, in a definite order which probably depends on the penetration of the drug into the substance of the ganglion. These successive trains of impulses can very easily be detected in the nerve trunks or muscles by the musical tone produced in the loud speaker, and a simultaneous note can be taken of the type of contraction evoked. That all the motor fibres are excited and then paralysed in this way is suggested by the complete reflex inexcitability of the preparation after the drug has taken effect, though the nerves can still be excited by electrical stimulation. The nomenclature adopted for the nerves of the metathorax is explained by Fig. 1, and the distribution of nerves to the muscles of the pleuron and coxa is shown in Fig. 6. The results are summarized in Table I. Table I Nerve 3? 3* Muscles supplied Quick and slow fibres to depressors and remotors of the coxa Two quick fibres to flexor trochanteris One slow fibre to reductor femoris One quick, one slow fibre to extensor tibiae One slow fibre to rotator coxae One quick, one slow fibre to pleural extensor trochanteris One quick, one slow fibre to extensor trochanteris Quick and slow fibres to flexor tibiae, depressor tarsi, and depressor metatarsi Supply to levator coxae muscles Two quick, several slow fibres to flexor trochanteris Discussion of the wnervation Of the four nerves running to the leg, two (4 and 6) are purely motor, and two (3 and 5) contain sensory fibres. Nerve 3 b supplies the outer coxal hair plate (Pringle, 1938 c) and several endings on the coxa; its sensory supply does not seem to extend below this segment. The majority of the sensory fibres from the leg run in nerve 5. Several features are worthy of special mention: (1) Some muscles are supplied by more than one quick fibre. Where this is the case, it is always obvious from close examination that a different part of the muscle is supplied by each fibre. In everything except function, the two portions behave as separate muscles. (2) The flexor trochanteris muscle is innervated from two different nerve trunks, 3 b and 6. The two portions of the muscle are quite distinct, the ventral portion innervated from 3 b being consistently of a redder colour than the rest. Similarly the extensor trochanteris complex is innervated from nerves 4 and 5, the supply from

12 The Motor Mechanism of the Insect Leg being limited to the pleural portion of the muscle. These cases, particularly the first, provide an interesting embryological problem, for the two nerves to the same functional unit are separated by many other muscles and cannot have been derived by splitting from the same nerve trunk. (3) In general, nerves 4 and 5 supply depressor muscles, and nerves 3 b and 6 levator muscles. The mobilization of muscle in the living insect We are now in a position to consider how the leg muscles of the cockroach are used in normal life. In the standing position a steady discharge of impulses is passing down the slow fibres to the depressor muscles of the leg, producing a tonic contraction which raises the animal off the ground. The variations in the frequency of this discharge with various types of sensory stimulation will be discussed in a later paper. On this background are superimposed short high-frequency bursts of impulses in the quick fibre, producing active contractions of the muscles and moving the animal over the ground. Slow movements of the insect such as are necessary for purposes of orientation and in moving round an object of food are achieved entirely by means of the slow fibre. The use of the quick-fibre mechanism, with its uncontrollable speed of contraction, seems to be limited to the rapid running of the insect in response to strong or noxious stimulation. It is therefore physiologically justifiable to distinguish "running" and "walking". SUMMARY 1. A double motor innervation is described from an insect muscle, the two nerve fibres producing respectively a tonic contraction and a twitch. 2. No evidence has been found for the existence of inhibitory fibres. 3. The nature of the two types of contraction is discussed, and compared with the similar double motor innervation of some Crustacean muscles. 4. The distribution of motor fibres to the muscles of the metathoracic leg of Periplaneta is described in detail. 5. The method of utilization of the leg in normal life is discussed. REFERENCES BUDDENBROCK, W. v. (1920). Pfliig. Arch. get. Pkytiol. 185, i. CREMER, E. (1935). Zool. Jb., Abt. Zool. 54, 191. FRIEDRICH, H. (1933). Z. vergl. Pkytiol. 18, 536. VAN HARREVHLD, A. & WIERSMA, C. A. G. (1937). J. exp. Biol. 14, 448. HBIDERMANNS, C. (1931). Zool. Jb., Abt. Zool. 60, 1. KRABMER, K. (1932). Zool.Jb., Abt. Zool. 51, 321. LANGLEY, J. N. & DICKINSON, W. L. (1889). Proc. roy. Soc. B, 46, 423. MAGNAN, A. (1924). he Vol det Intectes. Paris. MANGOLD, E. (1905). Z. allg. Pkytiol. 5, 135. MARCU, O. (1929). Anat. Anz. 67, 369. MATTHEWS, B. H. C. (1928). J. Pkytiol. 66, 225. PANTIN, C. F. A. (1934). J. exp. Biol. 11, 11. PRINOLE, J. W. S. (1938 a, b, c). J. exp. Biol. 15, 101, 114, 467. RIJLAND, P. (1932 a, b). C.R. Soc. Biol., Paris, 111, 631, 636. SANCHEZ, D. y S. (1913). Trab. Lab. Invest, biol. Univ. Madr. 11, 113. SNODGRASS, R. E. (1935). Principlet of Insect Morphology. New York. SOLF, F. (1931). Zool.Jb., Abt. Zool. 50, 175.