The Integument and Moult Cycle of Tegenaria atrica (Araneae)

The integument of Tegenaria atrica consists of two chitinous layers, an outer exocuticula and an inner endocuticula. The former is impregnated with protein and pigment and may be birefringent. Its thickness and development of birefringence is related to the hardness of each region. The exocuticula is formed before, and the endocuticula after, the moult. The hypodermis appears to secrete the chitin of both layers. The substances impregnating the exocuticula are probably brought to it by granular blood cells. These granulocytes remain in the hypodermis after the moult. The granulocytes responsible for the exocuticular secretion at the last moult migrate on to the old integument as this separates from the hypodermis at the next one. An ecdysial fluid is present and is absorbed by the time of the actual moult but very little digestion of the old integument occurs. The chromatin of the hypodermal nuclei increases in amount up to the beginning of the secretion of the new integument. It then sharply decreases until after the moult, when the cycle is resumed. There are three types of blood cell—granulocytes, leucocytes, and leberidocytes. The latter have a single large vacuole and are formed from the leucocytes. They only appear in relation to the moult, forming 65 % of the total blood cells immediately after it. The digestive diverticula secrete a fluid which fills most of the alimentary canal at the time of the moult. The above results are discussed. There is evidence that the relative hardness of the exocuticula is due to both impregnation with proteins and phenols and to changes of molecular structure. It is suggested that the leberidocytes absorb water from the food, via the plasma, to increase the blood pressure for moulting and subsequent enlargement of the animal. The filling of the alimentary canal with fluid is associated with this. The possible origin of a moulting hormone is considered.