Differentiation in the honeybee larva : a histological, electron-microscopical and physiological study of caste induction in Apis mellifera mellifera L.

In the first chapter, the constitution of larval food and the part this food plays in inducing caste differentiation are discussed. Postembryonic development as expressed by larval growth and moulting are vizualized in figs 1 and 2.

In Chapter 2, data on experimental animals (Table 1) and histological methods (Table 2) are presented.

In Chapter 3, contact chemoreceptors located on the labium and the maxillae are described. These sensilla appear to contain salt and sugar receptors. The possible role of the sensilla in discriminating larval food is mentioned.

In Chapter 4, fat body development in queen and worker larvae is described. Contrary to general opinion fat cells of the worker larva during the first three days appear to contain little stores. During this period worker larvae receive WJ. In the next period - when larvae are fed MWJ - accumulation of glycogen and fat occurs. Thus, there are indications that WJ restricts feeding. Large vacuoles in the fat cells of the (young) worker larva appear to have a watery content. Many of these vacuoles disappear during the 5th instar. Larvae in queen cells - in a queenless colony - normally are fed RJ. Within 18 hours after transfer of 48-hour-old worker larvae into queen cells (grafting), the fat body stores considerable amounts of glycogen. Watery vacuoles disappear and the amount of fat increases markedly: numerous spherical fat vacuoles appear in the cytoplasm. By then hardly any glycogen is present. In the fifth-instar queen larva, glycogen is being accumulated again. These observations suggest that worker larvae, unlike queen larvae, are undernourished during the first three days of postembryonic development.

In Chapter 5, the endocrine system of the larva is described. Little information is available on the part the different endocrine centres play in caste differentiation. Histology and electron-microscopy reveal cyclic activity in the CA of larvae of both castes. The CA in the queen larva seems to reach a high level of secretion within 24 to 36 hours after grafting.

In Chapter 6, the relation of JH to caste differentiation is discussed. JH titres in the haemolymph of queen larvae appear to rise within 28 hours after grafting, if compared with worker larvae of the same age. RJ seems to activate the endocrine system in some way. Topical application of JH on 3-day-old worker larvae results in the development of queenlike adults in worker cells. Thus, convincing evidence is present that JH is involved in caste differentiation.

In Chapter 7, some data are presented on the development of the fat body and the CA in larvae reared in vitro. The results suggest: 1. that mandibular gland secretion induces a worker-like type of fat body; 2. that RJ upon storage quickly loses its differentiation-promoting capacities. The CA does not increase in size markedly. Accumulation of food in the fat cells of larvae reared in vitro on fresh RJ, however, may be similar to that in colony-reared queen larvae.

In Chapter 8, present knowledge on caste differentiation in the honeybee is discussed.