Pierre Cassier

The Corpora Allata of Insects

Publisher Summary This chapter discusses the corpora allata of insects. Insect corpora allata or corps allates are endocrine glands which were previously called “paired posterior visceral ganglia,” “ganglia allata,” or “corpora incertae,” owing to their being confused with the sympathetic cerebral structures constituting the stomatogastric system. The endocrine activity of the corpora allata and the mode of action of the different forms of juvenile hormone (JH) are well known and retain a central position in entomological research. They are concerned with morphogenesis, activation of follicular cells and sexual-accessory glands, control of polymorphism, general physiology, and behavior. Research has been carried out on juvenoids, growth regulators, and juvenile hormone mimetics with specific short-range action affecting insects which are economically important. In small insects the corpora allata contain a limited number of cells and growth depends exclusively on cytoplasmic and nuclear hypertrophy which may or may not be associated with endopolyploidy.

Pheromonal activity and fine structure of the mandibular glands of honeybee drones (Apis mellifera L.) (Insecta, Hymenoptera, Apidae)

Abstract Experiments were conducted to determine the role of drone mandibular gland secretions in attracting flying drones and the effect of age on the secretory activity. Extracts of mandibular glands and of cephalic tissues were applied to cotton lures which were attached below balloons tethered at 8–12 m above the ground. Most flying drones were attracted to extracts of mandibular glands but a few were drawn to other cephalic tissues or to solvent controls. Histological and electron microscope studies showed that the structure of the tiny (0.12 mm long) mandibular gland varied according to age. Its secretory activity in 0–3-day old drones was evident from the abundant rough endoplasmic reticulum. At 7-days the glands were fully developed. After 9 days the glands were no longer active and showed an autolytic process; the product was stored in the gland lumen for further emission during drone mating flights.

Juvenile Hormone inhibition in corpora allata from ovariectomized Blattella germanica

Abstract. Ovariectomy has been used to study the role of the ovary in endocrine homeostasis. Our studies on young virgin adults of the cockroach Blattella germanica (L.) (Dictyoptera, Blattellidae) show that the cytological development of the corpora allata (CA) in ovariectomized females proceeds as in intact specimens, whereas the rates of Juvenile Hormone (JH) synthesis are lower. Stimulation of the CA from ovariectomized females in vitro by mevalonolactone suggests that enzymatic mechanisms which follow mevalonate formation in the biosynthetic pathway are functional. The synthetic capabilities of these CA are also illustrated by the kinetics of JH production in vitro, because hormonal release increases with time to reach ‘normal’ levels after 8h of incubation. Our data suggest that the absence of ovaries leads to effective inhibition of JH biosynthesis rather than to an impairment of the developmental process in the CA cells.

Haemolymph ecdysteroid titres controlled by a circadian clock mechanism in larvae of the wax moth, Galleria mellonella

Abstract The underlying mechanism of circadian rhythmicity of ecdysteroid titres in the haemolymph of the last-instar Galleria mellonella larvae was investigated. It was found that brain removal or head-ligature of larvae in which developmental processes were synchronized by transfer from 18 to 30°C, did not abolish daily rhythms of ecdysteroid titres in the haemolymph. Daily rhythms of ecdysteroid titres were also found in larvae ligated behind the thorax. The observed rhythms persisted 5 days after treatment, and transfer from 18 to 30°C, which suggests that the circadian clock driving these rhythms is not located within the brain. In further experiments it was shown that the prothoracic glands taken from temperature-synchronized larvae exhibit a daily rhythm of ecdysteroid synthesis with maximal activity coinciding with the peak of ecdysteroid titre found in the controls. It is therefore suggested that the circadian rhythm of ecdysteroid titres in Galleria larvae is regulated by a clock that probably is located within the prothoracic glands.

Control of corpus allatum activity during the imaginai diapause in females of Locusta migratoria L

In the Savio strain of Locusta migratoria an imaginai diapause is induced by long daylength. In diapausing females, the haemolymph level of juvenile hormone (JH) was undetectable during the first 3-wk of imaginai life and later rose only slightly to about 20 ng/JH3IR per ml. Only peripheral cells of the corpora aliata (CA) were active. In nondiapausing animals, or after the termination of diapause, the JH level was high (140–200 ng/ml) and the ultrastructure of the gland exhibited signs of activity. CA severance in 3-wk-old diapausing females terminated diapause as a result of activation of the CA. CA disconnection in the fifth larval instar or at the imaginai moult in long daylength animals did not break diapause and the CA stayed inactive. The lateral cells of the protocerebrum exert a jdual effect: at the end of larval life they bring about CA maturation and render them active, whereas during the imaginai diapause they inhibit CA activity. The median neurosecretory cells of the pars intercerebralis sup...

The sting sheaths of honey bee workers (Apis mellifera L.): Structure and alarm pheromone secretion

Abstract To evaluate the contribution of the sting sheaths of the worker to the defensive behaviour of a bee colony, we studied their fine structure and the effect of their secretions on the reaction of guards. (1) The structure of the proximal part of the sting sheath integument has all the characteristics of a primitive exocrine gland: porous cuticle with enlarged pore canals and epicuticular pores, and hypertrophied epithelial cells secreting an electron dense material. The non-volatile part of the secretory product embedded the setae of the sheaths. (2) Individual recipient guards in the laboratory and as a group at hive entrances in a beeyard displayed defence behaviour only when stimulated with either sting sheaths or with setaceous membrane, but not with any other gland or organ. Enhanced defensive reaction of guards has been observed when a venom gland or Koschewnikows glands, are added to either sting sheaths or to setaceous membrane. (3) We demonstrate for the first time that the sting sheaths have the structure of an exocrine gland and that their secretions induce defensive behaviour in guards.

Induction of choriogenesis by 20-hydroxyecdysone in the German cockroach.

Experiments in vitro have shown that 20-hydroxyecdysone (at a concentration of 0.2 and 2 microM and after 12 and 24 hr of incubation) is able to induce the precocious deposition of chorion materials by the follicular epithelium of young oocytes of the cockroach Blattella germanica. Since previous studies had shown that 20-hydroxyecdysone levels in B. germanica ovaries increase with oocyte maturation to reach a peak just before oviposition, we therefore hypothesize that ovarian ecdysteroids trigger Choriogenesis in this species through an autocrine action.

Ultrastructural changes induced by precocene II and 3,4-dihydroprecocene II in the corpora allata of Blattella germanica

SummaryUltrastructural studies on corpora allata (CA) from different stages during the first gonadotropic cycle of the cockroach Blattella germanica have shown well defined changes which have a correspondence with oocyte length, CA volume and juvenile hormone (JH) biosynthesis. The most significant variations concern the mitochondria and the endoplasmic reticulum. Topically applied precocene II (P II) at a dose of 200 ⧎g induced a transient arrest of CA function, although cytotoxic effects were occasionally observed. When CA were maintained in vitro with 10-3 M of P II, a relationship between the time of treatment (3, 6 or 9 h) and the intensity of the effects was apparent. The 9-h treatment led to an irreversible inhibition of JH production which parallels the severe damages observed in the CA (membrane lysis, nuclear pyknosis, vacuolization). Equivalent studies performed with the chroman derivative 3,4-dihydroprecocene II (DHP II) showed that it is less active than P II. Only treatments as severe as 12 h of incubation with a 10-3 M concentration elicited cytotoxic effects which could be due to radical species involved in the in situ oxidative bioactivation of DHP II. Thus, this compound could be regarded as a new type of pro-allatocidin.

The fine structure of the tarsal glands of the honeybee Apis mellifera L. (Hymenoptera)

SummaryTarsal glands are located in the 6th tarsomere of adult honeybee queens, workers and drones. Their structural features are not cast or sex specific. The glandular epithelium is lined by a thin endocuticular layer. A cuticular pocket is formed from a postimaginal delamination of the cuticle secreted by the glandular epithelium. The apical plasma membrane of the glandular cells shows numerous cristae and microvilli lining large crypts that communicate with the subcuticular space. Pinocytotic vesicles, multivesicular bodies and residual dense bodies are present in the apical part of the glandular cells. The RER is well developed in perinuclear and basal parts of the glandular cells, but the Golgi apparatus is a discrete organelle without secretory granules. No exocytotic secretory structures were observed. To reach the glandular pocket, the non-proteinaceous secretory product must pass across the subcuticular space, the cuticular intima, the space between the intima and the cuticular wall, and the cuticular wall of the glandular pocket.

The setaceous membrane of honey bee (Apis mellifera L.) workers' sting apparatus: Structure and alarm pheromone distribution

Abstract To evaluate the contribution of the setaceous membrane of the worker to the defensive behaviour of a bee colony, we studied its fine structure and the effect of alarm pheromones present on the membrane surface on the reaction of guards. Groups of guards at the hive entrances displayed defence behaviour when stimulated with volatiles from the setaceous membranes. Transmission electron microscopy sections of the setaceous membrane did not reveal any structure characteristic of an exocrine gland, in contrast to the sting sheaths or the Koschewnikow glands. Both surfaces of the membrane are abundantly covered with setae, which can considerably increase the general surface of the membrane and thus are suited for a quick discharge of alarm pheromones. The setaceous membrane serves only as a platform to release the alarm pheromones originating from the glandular part of the sting sheaths and the Koschewnikow glands.