The nutritional environment of honey-bee larvae affects the juvenile hormone (JH) titre of larval haemolymph and tissues. In this investigation the mechanism for the regulation of caste differentiation has been studied.
Chemo- and mechanoreceptors are found on larval mouthparts. Chemoreceptors on maxillae and labium are innervated by 5 bipolar neurons, making contact with the larval environment through one pore. Labral chemoreceptors are innervated by 4 bipolar neurons, probably also making contact with the environment through one pore.
All chemoreceptors studied are sensitive to sugars and salts. No sensitivity to other separately tested food components is found. This result is confirmed when considering receptor responses generated by diluted worker jelly (WJ) and royal jelly (RJ). Thus larval food components other than sugars and salts may not affect directly caste differentiation. Sugars possibly function as phagostimulants, regulating the amount of food consumed. The rate of food intake is considered to be the releasing factor for corpus allatum (CA) activity. Possibly neural stimuli arising from stretch receptors on the oesophageal or abdominal wall or indirectly from the brain may be involved.
Salts possibly function as inhibitors of the food intake. However, when adding salts to the food of queen larvae in the colony, the larvae leave the food deposited on the bottom of the cell. Food consumption is not restricted to the present food, but larvae receive also directly offered food from nurse bees. Therefore inhibition of food consumption by addition of salt in colony experiments resulted in a small number of intermediates only. Inhibition of food intake by starvation did not improve results to a large extent. A high rate of mortality and a small number of intermediates were obtained. Though not conclusive, some indication exists that limitation of food intake during the first three days of larval life, inhibits queen induction.
The neuroendocrine system of the larva corresponds in its main features with the classical pattern. In the pars intercerebralis 12 MNSC on both parts of the brain are found. Their axons innervate for one part the corpora cardiaca (CC) and for the other the CA, transporting neurosecretory granules, synthesized in the MNSC. Neurosecretory granules are accumulated in axon endings in the CC and in the neurosecretory fibres within the CA. Release phenomena are observed in the axon endings in the CC. CA are also innervated by two other axons. These axons never contained neurosecretory material throughout larval development.
Ultrastructural studies of CA in younger larvae reveal cycles of synthetic activities. MNSC however show little differentiation at that time. MNSC become active in larvae from about 60 hours of age. At the end of larval development CA innervating axons from the MNSC (nervus corporis allati or NCA) contain considerable amounts of neurosecretory material. Therefore MNSC can not be considered to trigger CA activity. Studies of synthesis, transport and secretion of elementary granules correlated with synthetic activities of the CA rather suggest that the neurosecretory substance inhibits CA activity.
These results are investigated further in detailed studies on MNSC and CA after changing the food of worker larvae into RJ and after application of JH to worker larvae. The activities of MNSC and CA are studied with quantitative electron- microscopical techniques. The effect of the change of food on the activity of MNSC and CA was determined after different periods. The change of larval nutrition appears not to affect the activity of the MNSC, but to stimulate CA activity. Prior to the increase of the CA activity pinocytosis at the periphery of the CA is observed. This process seems to be responsible for the increase in volume of the CA after intake of RJ by the larva. However pinocytosis is not considered as the trigger for the CA activity. Other processes not observed with the electron microscope might precede this phenomenon.
In JH treated worker larvae MNSC become activated at first. Increased amounts of neurosecretory material can be observed in the NCA, which coincides with the inhibition of CA activity. The decrease in CA activity after JH application can be explained by a feed back mechanism via the MNSC.
Results and conclusions are interpreted and discussed, resulting in a possible scheme of the mechanism of the caste differentiation in the honey bee.