The Later Embryology of Rhodnius prolixus

ABSTRACT The development of the external features of the embryo from the time at which the germ-layers are laid down until hatching is described. The fate of the embryonic membrane is followed, and the formation of a ‘cuticular’ layer round the whole embryo produced by the glands which result from the modification of the first abdominal appendages (pleuropodia). The central nervous system is produced in a similar manner to that of other insects. Two parallel longitudinal rows of neuroblasts separate from the dermatoblasts and give rise to the longitudinal nerve-cords. In between them is the neural groove, and cells dorsal to the neural groove produce a median cord. Neuroblasts in the protocerebral, antennary, and pre- mandibular segments form the supra-oesophageal nerve-mass, and those in the mandibular, maxillary, and labial the sub- oesophageal mass. At first three thoracic and eight abdominal ganglia are formed. Later concentration occurs, and by 18 days all the ventral ganglia are located in the thorax, the metathoracic ganglia being particularly large. The sympathetic nervous system arises from the dorsal wall of the stomodaeum. The corpus allatum originates as a pair of ectodermal cellmasses which appear to be invaginated from the maxillary segment. The ectodermal invagination to form the tentorium, glands, and spiracular system is described. Oenocytes appear previous to the ectodermal cells producing their cuticular layer. They become more numerous and much smaller after cuticle formation. The fore- and hind-gut develop normally. The Malpighian tubules are developed from ectoderm. The mid-gut arises in three parts, and the yolk becomes restricted to the middle part in the late embryos. From the mesoderm the usual structures are derived. Musculature of limbs and body is of two distinct kinds. The heart, fat-body, and splanchnic muscles are derivatives of the mesoderm. The germ-cells are defined before the germ-band invagi- nates. Later they are to be found, round the coeloms of the posterior abdominal segments. They migrate anteriorly and form complete gonads 10 days before hatching. The structure of the head of Rhodnius supports the orthodox view that the head of insects is composed of six segments. Evidence from the development of Rhodnius suggests that the insect labrum represents a pair of appendages. Time-table of development at 21° C. and 90 per cent, relative humidity.