VENTROMEDIAL NEUROSECRETORY NEURONS ( TYPE A ) IN THE SUBOESOPHAGEAL GANGLION OF MORIMUS FUNEREUS MULS . ( 1869 ) DURING POSTEMBRYONIC DEVELOPMENT

The presence of one pair of ventromedial neurosecretory neurons (vmnsn, type A) has been described in the suboesophageal ganglion (SG) of many insect species from different orders (Raabe, 1982). These neurons synthesize arginine-vasopresin neuropeptides, which have a diuretic effect on the Malphigian tubules and/or inhibit water resorption in the rectum (Philipps, 1983; Spring, 1990). Investigations of temperature stress in 2 and 3 year old larvae of Morimus funereus confirmed the presence of one pair ofvmnsn SG and brain neurosecretory neurons belonging to type A. Changes of their activity were noticed already after six hours of exposure to unfavorable temperature, which points to a significant role of its neurohormones in the processes of acclimation and acclimatization to thermal stress (Iv an o v i c eta!., 1975a, 1975b).

The presence of one pair of ventromedial neurosecretory neurons (vmnsn, type A) has been described in the suboesophageal ganglion (SG) of many insect species from different orders (Raabe, 1982).These neurons synthesize arginine-vasopresin neuropeptides, which have a diuretic effect on the Malphigian tubules and/or inhibit water resorption in the rectum (Philipps, 1983;Spring, 1990).Investigations of temperature stress in 2 and 3 year old larvae of Morimus funereus confirmed the presence of one pair ofvmnsn SG and brain neurosecretory neurons belonging to type A. Changes of their activity were noticed already after six hours of exposure to unfavorable temperature, which points to a significant role of its neurohormones in the processes of acclimation and acclimatization to thermal stress (Iv a n o v i c eta !., 1975a, 1975b).
Investigations of differentiation of the cerebral neuroendocrine system during embrionic development of M funereus showed that 6 to 7 day old embrios had a general morphological plan of cerebral complex structure.A large number of paraldehyde-fuchsin positive (PF+) neurosecretory neurons (type A) in various phases of differentiation were found in the brain, sub oesophageal ganglion, and frontal ganglion.The highest level of differentiation was noticed in SG, which underlines the functional and evolutionary significance of its neurohormones (St an i c eta !., 1978;Nenadovic et al., 1986).
The present work gives the results of investigations of changes in the size and number ofvmnsn SG during postembrionic development of M.funereus.
The suboesophageal ganglion of newly hatched larvae is still more developed than the brain.Brain lobes are feebly differentiated and distant.As flattened foils, they lean against the gut in such way that their lateral parts are in a ventral position.Corpora allata are well-developed, while the corpus cardiacum is insufficiently differentiated and has a small number of visible axons and intrinsic neurons, which is characteristic of embrios (N e n a d o v ic et a !., 1978, 1987).
The suboesophageal ganglion of newly hatched larvae is characterized by well-developed neuropil and a large number of PF+ neurosecretory neurons.Embrionic traits are less expressed in SG than in the brain, i.e., neurons of SG are more differentiated than brain neurons (N enadovi c et al., 1985; N c n ad o v i c , 1992).
In view of the long postembrionic developmnet of M.funereus, the question arises as to how long cytodifferentiation of PF+ neurosecretory neurons of its SG lasts.

3P
The M..funereus larvae used in this experiment were reared in laboratory conditions from hatching to sacrificing (constant temperature of 23°C, relative humidity of 70%, absence of light, and artificial diet according to Roberts, 1986).Under these conditions, larval development lasts 6.5 months and pupal development 15 days (Nenado vi c et a!., 1989;St an ic eta!., 1989;Nen ado v ic , 1992).Individuals were sacrificed on the fourth day after molting into the first, third, fourth, sixt, and eighth larval instars (the eighth being the last larval instar, LU); in the prepupal and pupal stages; in pharat adults; and in 24-h adults.Each group consisted of five individuals.Head capsules were fixed in Bouin's fixative.Standard histological procedure was used for embedding in paraffin.Serial paraffin sections of 5urn were stained using the paraldehyde Thionine Phloxine and Alcian Blue Phloxine techniques (Panov, 1980).Tissue analysis was performed using a light microscope.The size of pericarya was expressed as average values of the product of their larger and smaller diameter.
The results presented on Fig. 1 show increase in size of vmnsn from the first to eighth larval instar (LU).The size of these neurons decreases in the prepupa and pupa, i.e., at the beginning of and during metamorphosis.The size of vmnsn SG increases in adults and they become larger than larval neurons.According to P anov (1968), the synthetic activity of neurosecretory tissues can increase by increase in number and/or size of their neurons.Both mechanisms have been described for protocerebral neurosecretory neurons of type A (AI' A 2 , L" L 2 etc.) in Mfunereus (Ne n ado v i c , 1992).Similar results have been obtained in Tenebrio molitor (Janko v i c-Hl adn i , 1971), which is in agreement with results obtained in other insect species (Satija and Kaur, 1965;Weeks and Levine, 1990).
A high number ofPF+ vmnsn SG is evident at the beginning of the first larval instar (which lasts 6-7 days), but there were only three pairs of neurons (one pair of dorsal and two pairs of lateral neurons) on the fourth day of the first instar.Their number and position did not change during further postembrionic development.There are two possible causes of decrease in number of vmnsn SG in the first instar of M. funereus larvae: death or migrations of neurons.Neuron death is best investigated in insect metamorphosis (Singh and Srivastava, 1974;Schwarc and Truman, 1982), but it is also known to happen in specific phases of embrionic and postembrionic development under the influence of many exogenous and neurohormonal fac-tors (Fahrbach, 1997).Migrations of PF+ neurons to other parts of central nervous system also appear to be possible, in view of the presence of arginine-vasopresin like neurons in the protocerebrum and first thoracic ganglion of some insects (Girardie et al., 1984).Since diuresis in insects is under neuroendocrine control (Spring, 1990), further investigations on vrnnsn SG would be of great importance for solving both theoretical and practical problems.