The anatomy of the nervous system in the genus Gerris (Hemiptera-Heteroptera)

Abstract

The most successful methods used in this study were Palmgren’s silver-on-the-slide technique, the rapid silver nitrate method of Golgi, and a method that has seldom been applied to insect material, the Golgi-Cox mercuric chloride method. A way of preparing sections of adult hardened insects by infiltrating with wax prior to softening with chlorinated acetic acid and nitric acid was also employed. The nervous system of Gerris shows a high degree of condensation in that all the segmental neuromeres are fused. This characteristic of the nervous system may be associated with the disposition of the one hundred and twenty-two pairs of muscles. The nerves of the head are specialized in association with the complex mouthparts. There are distinct stylet and labral ganglia, and peripheral interconnexions between some of the nerves. The posterior labral nerve was traced to the principal salivary gland. The reticulum described by Baptist (1941) as of a nervous nature was shown to consist of fine muscle fibres: the much finer nerve fibres were also stained in silver preparations. The many separate nerves of the prothorax reflect the unspecialized nature of this segment as compared with the meso- and metathorax in which most of the fibres are gathered into a few nerve trunks. The coalesced neuromeres of the abdominal region give rise to a pair of posterior nerve trunks connected with small ganglia or lateral bodies lying near the spiracles. These ganglia appear similar to the structures described by Landois & Thelen (1867), as controlling spiracular movements in Cossus . Rough estimates of the number of cells in different parts of the nervous system were correlated with the percentage success of staining methods, and specialization of the neuromeres. The form and arrangement of neurones within the optic and protocerebral centres of Gerris conforms for the most part to the patterns worked out in other insects, though there do not seem to be as many different types of internuncial neurone in the optic lobes of Gerris as exist in Apis or Calliphora (Cajal & Sanchez 1915). The corpora pedunculata are connected through a dorsal glomerulus with the deutocerebrum, the glomerulus having the form of a loose meshwork of fine fibres rather than of a distinct calyx. The deutocerebrum is indistinctly divided into anterior and posterior glomeruli, as described in Apis by Sanchez (1936). The form and size of the elements composing the somewhat enigmatic posterior glomerulus in Gerris supports the view that this is a motor centre. The close association between the maxillary and mandibular nerves is to some extent reflected in the internal organization of these neuromeres. Separate ventral areas could be distinguished, but ganglionic boundaries were indistinct. The large labral centre shows many of the features of a trunk ganglion. The pattern of neurones in the thoracic and abdominal centres could be compared in detail with the pattern described by Zawarzin (1924) in the larva of Aeschna . There are three unusually large internuncials with processes in this region, and cell bodies in the protocerebrum and suboesophageal centres. It is suggested that they form part of a dual physiological system controlling the motor centres of the thorax (Roeder 1953). The mesothoracic centre was made a special object of study as representative of the thoracic neuromeres. The fibre tracts are clearly marked and can be seen to correspond to functional regions within the centre. The alary nervous system of the mesothorax was investigated in some detail in both winged and wingless forms of Gerris . In the flying forms dorsal and ventral tracts can be distinguished, associated with motor and sensory regions of the mesothoracic neuropile respectively. In forms without wing muscles or fully developed wings the dorsal tract is absent or vestigial and the ventral tract is clearly reduced. The abdominal neuromeres are very closely compacted so that they tend to lose their identity. The ventral longitudinal tracts are unusually well developed and this may be correlated with the importance of the sensory areas.