The Embioptera are rather generalized insects whose internal anatomy is simple and not subject to great modifications. For this reason these insects form an ideal group for elementary anatomical and histological studies (fig. 2). The digestive tract is a long, simple tube without convolutions or diverticulae from the buccal cavity to the rectum. The buccal structures are of the chewing type. The oesophagus and ingluvia are differentiated only by slight dilation of their walls. In nymphs and females the proventriculus is very distinct due to folds which flatten as the structure becomes packed with food. The enteron is the largest in such forms and in both sexes limited caudally by the Malpighian tubules. The proctodeus has six large rectal papillae. The nervous system is complete with only the fifth abdominal segment lacking a ganglion in the metathorax includes the ganglion of the first abdominal segment. The brain exhibits very clear structure in histological sections. The tracheal system includes two pairs of thoracic spiracles and eight abdominal pairs. Only th metathoracic spiracle has an air expiration function; all others serve for inspiration. Various structures in the spiracles protect the atrium. The circulatory system includes a long, simple dorsal vessel which extends forward from the ninth abdominal segment into the cranium. It opens anteriorly near the circumoesophageal connectives. The dorsal vessel has a pair of ostia and valves corresponding to each abdominal and thoracic segment. It lacks the diverticulae or folds commonly found in more highly evolved insects. The excretory system is represented by Malphighian tubules, pericardial cells, and fat-body. The number and disposition of Malpighian tubules is variable within the order. The pericardial cells are localized around the entire dorsal vessel up to the opening of the aorta in the head. The fat-bodies form compact layers in the dorsal and ventral regions of the body. In males they are more developed in the abdominal region. The mandibles, maxillae, and salivary glands are of a simple type with very few cytological modifications. Only the salivary glands which extend into the mesothoracic region show appreciable specialization. The reproductive system is bi-sixual and shows considerable sexual dimorphism. Males have five pair of testes with a metameric disposition, two distinct ducts, two epidymis, and the ejaculatory organs. The accessory glands vary in number and size and open in the anterior portion of the ejaculatory duct. The female reproductive organs are of the panoistic type. The system includes five pairs of ovarioles, two long paired oviducts a small, unpaired oviduct and the spermatheca which opens in the vagina. Reproduction usually involves a union of male and female gametes, and eggs are usually laid in clusters attached to a substrate.