On the Biology and Structure of the Larvae of Hydrophilus caraboides L. By E. N. Pavlovsky, M.D., D.Sc, Professor of Zoology at the Military Academy of Medicine, Petrograd. With Plate 27 and 16 Text-figures. IN May 1918 I captured in the vicinity of Petrograd some cocoons of a hydrophilus beetle, one of which I kept for breeding purposes. On June 13 there emerged about fifty small larvae very similar to Hydrophilus caraboides. These latter are characterized by the presence of a pair of lateral (pleural) appendages covered with a dense brush of hairs on each of the seven abdominal segments (Text-fig. 13, pla). In my larvae (Text-fig. 1) these pleural hairy appendages were also present, but with the difference that each appendage bore on its summit a long thin hair. This peculiarity caused me to look for other differences between my larvae and the description of the larvae of Hydrophilus caraboides, as given by Schiodte in his paper, ' De metamorphosi eleuthera- torum observationes; bidrag til insekternes undviklingshistorie', 1861. These differences may best be pointed out by a parallel com- parison of the text of Schiodte's diagnosis and the description of the newly emerged larvae as observed by me, as follows : Hydrophilus cara- Hydrophilus caraboides boides Schiodte Caput —a larva at first stage. Head obovatum. irregularly tetragonal, shaped rather like a trapezium turned with its base forward, and with broken sides. 628 E. X. PAVLOVSKY Antennae articulo primo Mrst joint of antennae long, longissimo, tenui, ciliato slender and flat, inner margin (Text-figs. 6, 7, V), secundo with seven very distinct ft tertio tenuibus, pusillis, teeth (Text-fig. 5, 7, a), giving subaequalibus. the joint a saw-like aspect. Second and third joints almost of equal length, each being about three times shorter than the first. Second in the middle of its outer margin with an elevation bearing a sense-organ in the shape of a chitinous ringlet with hairs in the centre. Antennae scapo longis- Antennae scapo longissimo, simo ciliato, natatoriae. serrato, masticatoriae. Mandibulao elongatae, Mandibles shaped as a sharp, falcatae, acutissimae, sub- bent, and strong sickle with aequales, dente medio secondary teeth on its concave valido, duplici, acumine, surface, the first largest, lancet posteriore acuto, priore lan- like ; the: middle tooth tetra- coolato, inaiore (Text-fig. 6, gonal with a concave free surface md). and sharp prominent edges ; hind tooth in the shape of a small tubercle at the base of the middle tooth. Stipites maxillares gracillimi, longis- simi, palpos labiales excedentes. Palpi maxillares graciles : stipes gracilis, re- curvus, articulo primo triplo longior. Mentum (Text-fig. 4, in) Mentum (Text-fig. 3, in) bell- ampluin, fornicatum, e basi shaped, both its fore angles pro- sensim, decrescens latitu- truding in the form of sharp dine, angulis dentiformibus, teeth. Side margins smooth, lateribus serrulatis. slightly S-shaped. Stipes com- Stipes palporum labialium pared with the mentum much t'longato-quadratus, basin more developed than in the versus angustior. Palpi mature larva of H. c a r a - labiales (pll) articulo priore bo ides, and in the form of an brevissimo. Ligulaelongata, elongate rectangle which in its ticuminata (Ig). fore part is wider than behind. The base of the ligula occupies almost half of the width of the anterior margin of the mentum (in the mature larva of H. c a r a - b o i d e s only one-fourth). Tip of theligula is somewhat bilobate (Ig). BIOLOGY AND STRUCTURE OF HYDROPHILUS CARABOIDES (i'29 Scuta prothoracica con- The prothoracic tergite has the creta, completa, integra. shape of a continuous shield of Scuta mesothoracica dis- an almost regular quadrangle, creta, tergum medium oc- the posterior corners being cupantia, triangula, apicem obliquely cut or rounded (the truncatum versus con- tergite of the mature larva of structo-attenuata. Scuta H. c a r a b o i d e s is conspicu- metathoracica discreta, in- ously narrowed in front and in completa, apice incurva. outline much like a truncated cone). The meso- and meta- thoraces bear each on their ter- gites two triangular shields, the apex of which is not bent. Praeterga abdominis in- Abdomen densely hairy (Text- tegra. Terga obscurius fig. 1). Tergites with four warty plicata, scutis verruciformi- tubercles which are also covered bus, cylindricis. Appen- with hair, and with a long seta dices pleurarum abdominis on the summit. Before the praeter octavum par elon- middle pair of tubercles there gatae, inaequales, ciliis is a pair of brown shields of gracillimis vestitae, n a t a - strong chitin, also bearing long toriae; appendices oc- setae. tavi paris deorsum flexae, Pleural appendages in shape acuminatae, sinuosae, sus- of cylindrical prominences, and pensoriae. densely covered with hair, resem- bling a lamp-brush. From the summit of each appendage there emerges a long and thin seta (Text-fig. 1). Above the base of each lateral pleural appendage there is a small warty tubercle with a long seta. The examination of more advanced larvae of Hydro- philus caraboides, i.e. such that had already moulted, convinced me of the accuracy of Schiodte's description. The antennae of such larvae are in fact hairy on their inner edges, and the base of the lamella of the under-lip has a dentate edge. The long setae on the tips of the pleural appendages are partially present but not conspicuous. On the whole one might say that from the egg of H y d r o - philus caraboides emerges a larva which in its successive 630 E. N. PAVLOVSKY ecdyses changes some structural characters, especially in the antennae, the under-lip, and partly the pleural appendages. In the literature the excellent drawing of the mature larva given by Schioclte is generally adopted ; but I have not been TEXT-FIG. 1. Hydrophi 1 us caraboides. A larva, first stage. able to find any figure of a young larva, therefore I here adduce its figure, drawn unfortunately from a specimen preserved in spirit, on account of which the abdomen seems shorter than it is in reality (Text-fig. 1). It is true that it easily extends, a fact which causes some variation in its length even under normal conditions. These facts found bv me caused me to investigate the larvae BIOLOGY AND STRUCTURE OF HYDROFHILUS CARABOIDES 631 of H y d r o p h i 1 u s c a r a b o i d e s more thoroughly. The larvae were brought alive into Duboscq's mixture (15 c.c. of 1 per cent, solution of picric acid in 90 per cent, alcohol + 6 c.c. of formalin and 1-5 c.c. of ac. aceticum glaciale), and after they had ceased to mo.ve they were cut in two with scissors. The fixation lasted from twelve to sixteen hours. After this the larvae were brought through several portions of 85 per cent, alcohol to wash out the picric acid. Further, they were embedded in paraffin. The sections were stained with iron haematoxylin, Giemsa stain, and Unna's polychrome methylene-blue. ON THE STRUCTURE OF THE DIGESTIVE APPARATUS. The larvae of H. caraboides are distinguished from all other aquatic insects by their unusual way of taking food. Observations on this point have already been made by several naturalists, and Brocher (1913) even figures (semi-diagramma- tically) a larva of H. caraboides taking food (fig. G8). According to a more detailed description of Portier (1911) this process takes place as follows. Having caught with its powerful and sharp mandibles Its prey (for instance a small crustacean, an insect, &c), the larva of Hydrophilus ' se dirige vers une plante aquatique on contre la paroi du vase qui la renferme, si aucun objet ne flotte a la surface de l'eau. Au moyen de ses pattes anterieures, elle s'accroche a une asperite quelconque situee au-dessus de la surface, puis elle renverse sa tete en arriere sur son dos. On comprend que, dans cette situation, sa tete emerge com- pletement. C'est, en effet, le resultat auquel la larve semble tendre. ... La proie ayant ete ainsi elevee au-dessus de la surface, on voit l'appareil masticateur entrer en jeu, la chitine est perforee par les tubercules chitineux qui garnissent les mandibules et le sang rouge de la larve de 0hironomus1 se met a couler. ' A ce moment, un flot de liquide noir envahit les organes 1 Portier describes the feeding of a larva of H. caraboides on a larva Chironom us. 632 K. N. PAVLOVSKY buccaux ; c'est le liquide digestif qui a ete injecte a travers l'cesophage par l'intestin moyen. Ce liquide noir adhere par capillarite a la proie ; il est contenu dans une espece de cor- beille formee par les differentes pieces de l'armature buccale TEXT-FIG. 2. Larva of Hydrophilus oaraboides devouring a oypris. si developpee chez ces larves. La proie est ainsi completement baignee dans la liqueur digestive. Apres quelques instants de contact, le liquide charge maintenant des produits de la digestion est reabsorbe ; il passe par l'oesophage, puis dans l'intestin moyen dont il gagne imrnediatement la partie pos- terieure, ainsi qu'on peut s'en assurer sur les jeunes larves, ou sur celles qui Viennent de changer de peau, toutes deux posse- BIOLOGY AND STRUCTURE OF HYDROPHILUS CARABOIDES 633 dant des teguments d'une transparence parfaite' (Portier, 1911, pp. 175-6). The inconvenience of the attitude during the feeding process (Text-fig. 2) is increased for the larva of H. caraboides by the necessity of breathing. For this purpose the larva bends its body in such a way that the posterior end of the abdomen TEXT-FIG.