Ultrastructure of the epidermal maxilla II-gland of Scutigera coleoptrata (Chilopoda, Notostigmophora) and the ground pattern of epidermal gland organs in Myriapoda

The epidermal maxilla II-gland of Scutigera coleoptrata was investigated using light and electron microscopy. The glandular epithelium surrounds a spacious integumental cavity at the base of the maxilla II. The gland is formed as a compound gland organ that is composed of thousands of epidermal gland units. Each of them consists of four different cell types: a secretory cell, an accessory or intermediary cell, and a proximal and distal canal cell. The intermediary and the two canal cells form a conducting canal. Only in the most distal part of the intermediary cell is the canal lined by a cuticle. In the area of the two canal cells, the conducting canal is completely covered by a cuticle. The canal passes through the cuticle and opens into the spacious integumental cavity, which serves as a secretion reservoir. The structural organization of the epidermal maxilla II-gland was compared to that of other compound epidermal gland organs in Chilopoda and Diplopoda. All these glandular organs in Myriapoda share the same ground pattern.     

Ultrastructural investigation of a salivary gland in a centipede: structure and origin of the maxilla I-gland of Scutigera coleoptrata (Chilopoda, Notostigmophora)

The maxilla I-gland of Scutigera coleoptrata was investigated using light and electron microscopy methods. This is the first ultrastructural investigation of a salivary gland in Chilopoda. The paired gland opens via the hypopharynx into the foregut and extends up to the third trunk segment. The gland is of irregular shape and consists of numerous acini consisting of several gland units. The secretion is released into an arborescent duct system. Each acinus consists of multiple of glandular units. The units are composed of three cell types: secretory cells, a single intermediary cell, and canal cells. The pear-shaped secretory cell is invaginated distally, forming an extracellular reservoir lined with microvilli, into which the secretion is released. The intermediary cell forms a conducting canal and connects the secretory cell with the canal cell. Proximally, the intermediary cell bears microvilli, whereas the distal part is covered with a distinct cuticle. The cuticle is a continuation of the cuticle of the canal cells. This investigation shows that the structure of the glandular units of the salivary maxilla I-gland is comparable to that of the glandular units of epidermal glands. Thus, it is likely that in Chilopoda salivary glands and epidermal glands share the same ground pattern. It is likely that in compound acinar glands a multiplication of secretory and duct cells has taken place, whereas the number of intermediary cells remains constant. The increase in the number of salivary acini leads to a shifting of the secretory elements away from the epidermis, deep into the head. Comparative investigations of the different head glands provide important characters for the reconstruction of myriapod phylogeny and the relationships of Myriapoda and Hexapoda.     

The anal glands of Rhapidostreptus virgator (Diplopoda: Spirostreptidae)

Summary In Rhapidostreptus virgator exocrine gland complexes are found in the anal valves of both sexes. Every gland complex consists of about 200 secretory units, each of which is comprised of four cells: two secretory cells, an intermediary cell, and a canal cell. The amount of secretion produced by these glands varies during the intermoult cycle: it is very small in freshly moulted individuals (postmoult phase), at a medial level during the following intermoult phase, and very large in the premoult phase. The secretion may be used to form the excrement clumps and above all to build the moulting chamber.     

Ultrastructural investigation of the vesicular glands in Scutigera coleoptrata (Chilopoda,Notostigmophora)

In the notostigmophoran centipedes, two pairs of vesicular glands have evolved. These paired glands are situated in the first and second trunk segment and open via cuticular ducts in the upper part of the particular pleura. The vesicular glands of Scutigera coleoptrata were investigated using light and, for the first time, electron microscopical methods. The glands consist of wide sac‐like cavities that often appear vesicular. The epithelia of both glands are identically structured and consist of numerous glandular units. Each of these units consists of four different cells: a single secretory cell, a small intermediary cell, and one proximal and one distal canal cell. The intermediary cell forms a conducting canal and connects the secretory cell with the canal cells. Proximally, the intermediary cell bears microvilli, whereas the distal part is covered with a distinct cuticle. The cuticle is a continuation of the cuticle of the canal cells. This investigation shows that the ultrastructure of glandular units of the vesicular glands is comparable to that of the glandular units of other epidermal glands in Chilopoda and Diplopoda, although the glands look completely different in the light microscope. Thus, it is likely that the vesicular glands and epidermal glands share the same ground pattern. With regard to specific differences in the cuticular lining of the intermediary cells, a common origin of epidermal glands in Myriapoda and Hexapoda is not supported. J. Morphol. 2009. © 2008 Wiley‐Liss, Inc.     

Location and ultrastructure of sex pheromone glands in female Callosobruchus maculatus (Fabricius) (Coleoptera : Bruchidae)

The ultrastructure of the female sex pheromone glands in Callosobruchus maculatus (Coleoptera : Bruchidae) were localized using a masking technique, combined with eiectro-antennography and by a comparison of the glandular cells of sexually active (flightless) females and non-sexually active (flight-form) females. Each unicellular gland is an invagination of the integumental membrane capped by a single secretory cell. These glands are situated on the fine intersegmental membrane, which joins the pygidium to the ovipositor. The secretory cells of the glands of active females are characterized by well-developed microvilli, with many elongated mitochondria among the latter. The high metabolic activity of these cells is revealed by the presence of heterogeneous secretion vesicles, some of which contain abundant crystallized material. Deep basal invaginations indicate the uptake of substances from the haemolymph. The receptor canal is a network of fine cuticular filaments which have the same structure regardless of the female's sexual status. Cells from the glands of non-sexually active females are underdeveloped and show no invaginations of the basal membrane and very few microvilli. The localization of these glands was made possible by the use of SEM, TEM and EAG as well as by masking the suspected zones and by comparing females in different physiological states: flightless females, which were sexually active and producing pheromones; and flight-form females, non-sexually active and producing no sex pheromones. Only by adopting such a stringent method was it possible to confirm the function of the glands whose ultrastructure was studied.     

An interommatidial exocrine gland with a “nail-headed” structure in the water strider Aquarius remigis (Hemiptera,Gerridae)

The fine structure of the interommatidial exocrine glands, found in the compound eyes of the water strider Aquarius remigis, is described using light, scanning, and transmission electron microscopy. The glandular pores of the glands are specialized into minute “nail-headed” structures (NS), which are described for the first time in arthropod compound eyes. Each NS is composed of two components: a rod-like stalk and a cup-like depression. The TEM study shows that the glands are class 3 epidermal glands as defined by Noirot and Quennedey (1974, 1991). Each gland consists of 3 cells: a gland cell, an intermediary cell, and a duct (canal) cell. The gland cell contains abundant electron-lucent vesicles, while the intermediary cell contains a large number of osmiophilic secretory granules. These two cells might secrete different substances which mix together in the dilated sac-like portion of the conducting canal before final release. The possible functions of the secretions released from these glands are discussed.     

The pygidial defense gland system of the Steninae (Coleoptera,Staphylinidae): Morphology,ultrastructure and evolution

The pygidial defense glands of the Steninae consist of two big (r1) and two smaller (r2) secretion filled sac-like reservoirs with associated secretory tissues and basal eversible membrane structures. The secretion is made up of deterrent and antimicrobial alkaloids stored in r1 as well as terpenes in r2. The gland cells filling r1 form a band shaped secretory tissue (g1) in an invagination of the reservoir membrane. The content of r2 is secreted by a tissue (g2) surrounding the efferent duct of r1 opposite to r2. In both gland tissues the secretion is produced in type IIIt gland cells and accumulates in an extracellular cavity surrounded by numerous microvilli of the gland cell membrane. After exocytosis the secretion enters an epicuticular duct and is transported to the corresponding reservoir via a conducting canal enclosed in at least one canal cell. While the structure of g1 is very similar in all species of the Steninae, g2 is often reduced. This reduction of the system r2/g2 is accompanied by a decreasing amount of terpenes in the total secretion and could be of interest for phylogenetic studies in the subfamily of the Steninae.     

黑缘烟蟋螽丝腺结构

【目的】蟋螽是直翅目中唯一具有吐丝筑巢行为的类群。本研究旨在探讨蟋螽丝腺的结构特点。【方法】应用解剖学观察、免疫荧光、苏木精-伊红染色、PAS苏木精染色、扫描电镜和透射电镜等方法从细胞水平对黑缘烟蟋螽Capnogryllacris nigromarginata丝腺的显微与超微结构进行了观察。【结果】黑缘烟蟋螽丝腺由导管和腺泡构成。腺泡由鞘细胞延伸形成的结缔组织鞘包围。腺泡的主体有4种细胞,分别为Ⅰ型分泌细胞、Ⅱ型分泌细胞、围细胞和腔细胞。Ⅰ型和Ⅱ型分泌细胞为大的腺细胞,形状不规则。分泌细胞细胞核很大,胞质内有大量的内质网和分泌颗粒。Ⅰ型分泌细胞靠近腺泡中心,PAS-苏木精染色表明Ⅰ型分泌细胞内含糖蛋白,Ⅱ型分泌细胞在腺泡外周,位于Ⅰ型分泌细胞与围细胞或结缔组织鞘之间。腔细胞分散在分泌细胞之间,包围形成胞外运输分泌物的通道。围细胞与鞘细胞接触,具有由细胞膜内陷形成的微绒毛腔,胞质内有大量的线粒体。围细胞微绒毛腔与腔细胞包围的细胞外运输通道相连,分泌细胞分泌的颗粒聚集在分泌细胞和胞外运输通道之间的连接处,并将分泌物排出至胞外运输通道。多个腺泡的胞外运输通道汇集到由单层细胞组成的丝腺导管。单层导管细胞靠近管腔外围具有规则排列的质膜内陷和大量伸长的线粒体;靠近管腔的一侧具连续的细胞膜突起,在导管壁的表皮下紧密排列。【结论】黑缘烟蟋螽丝腺分泌细胞分为Ⅰ型分泌细胞和Ⅱ型分泌细胞。分泌物质产生及分泌过程依次经过分泌细胞、腔细胞包围的胞外通道、分支导管、总导管和唾窦。其中在腺泡细胞之间,分泌物向外运输过程中,围细胞微绒毛腔的微丝束可能对分泌物的外排提供推动力。     

Rho GTPase controls invagination and cohesive migration of the Drosophila salivary gland through Crumbs and Rho-kinase

Coordinated cell movements shape simple epithelia into functional tissues and organs during embryogenesis. Regulators and effectors of the small GTPase Rho have been shown to be essential for epithelial morphogenesis in cell culture; however, the mechanism by which Rho GTPase and its downstream effectors control coordinated movement of epithelia in a developing tissue or organ is largely unknown. Here, we show that Rho1 GTPase activity is required for the invagination of Drosophila embryonic salivary gland epithelia and for directed migration of the internalized gland. We demonstrate that the absence of zygotic function of Rho1 results in the selective loss of the apical proteins, Crumbs (Crb), Drosophila atypical PKC and Stardust during gland invagination and that this is partially due to reduced crb RNA levels and apical localization. In parallel to regulation of crb RNA and protein, Rho1 activity also signals through Rho-kinase (Rok) to induce apical constriction and cell shape change during invagination. After invagination, Rho-Rok signaling is required again for the coordinated contraction and dorsal migration of the proximal half of the gland. We also show that Rho1 activity is required for proper development of the circular visceral mesoderm upon which the gland migrates. Our genetic and live-imaging analyses provide novel evidence that the proximal gland cells play an essential and active role in salivary gland migration that propels the entire gland to turn and migrate posteriorly.     

Fine structure of the tegumentary glands secreting the protective "shield" in a sessile insect (Homoptera, Diaspiddae)

The tegumentary pygidial glands of Aonidiella aurantii (Homoptera, Diaspididae) produce a secretion forming the shield of these fixed parasites of plants. They are formed of seven cells: a principal unpaired secretory cell which produces an abundant glycoproteinaceous secretion; a small associated cell with a secondary reservoir for this secretion; two accessory secretory cells which have very abundant tubular extensions coming from the plasma membrane, and a flocculent secretion gathered in a large sub-cuticular space; two cells forming an enlarged part of the excretory canal, functioning like a spinneret; and finally a single cell forming the tubular duct of this complex gland. The cuticle of the secretory cells has a very special porous structure, through which the secretion passes. The final product is a ribbon formed by two hollow strands stuck together. The exact nature of this secretion is not clear. It is comparable to a silk secretion though it has its own particular characteristics.     

Ultrastructure of an exocrine dermal gland in the gills of the grass shrimp,Palaemonetes pugio: Occurrence of transitory ciliary axonemes associated with the sloughing and reformation of the ductule

Exocrine dermal glands, comparable to the class 3 glandular units of insects, are found in the gills of the grass shrimp, Palaemonetes pugio. The dermal glands are composed of three cells: secretory cell, hillock cell and canal cell. Originating as a complex invagination of the apical cytoplasm of the granular secretory cell, a duct ascends through the hillock and canal cells to the cuticular surface. The duct is divisible into four regions: the secretory apparatus in the granular secretory cell, the locular complex, the hillock region within the hillock cell and the canal within the canal cell. A tubular ductule is contained within the latter two regions. As the ductule ascends to the cuticular surface, its constitution gradually changes from one of a fibrous material to one which possesses layers of epicuticle. During the proecdysial period, the ductule is extruded into the ecdysial space and this is followed by the secretion of a new ductule. Temporary ciliary structures, located near the secretory apparatus of the secretory cell, are associated with the extrusion and reformation of the ductule. Characterized only by a basal body and rootlets throughout most of the intermolt cycle, the ciliary organelles give rise to temporary axonemic processes which ascend through the ductule toward the ecdysial space at the onset of proecdysis. Subsequently, the old ductule is sloughed off and a new ductule is reformed around the ciliary axonemes. Following this reformation, the ciliary axonemes degenerate. The function of cytoplasmic processes, derived from the apical cytoplasm of the secretory cell, is also discussed.     

Ultrastructure of excretory system in <Emphasis Type="Italic">Bothrioplana semperi</Emphasis> (Platyhelminthes,Turbellaria)

The ultrastructure of flame bulbs and epithelium of excretory canals in Bothrioplana semperi (Turbellaria, Seriata) have been studied. The flame bulbs consist of two cells, the terminal cell and the proximal canal cell. The weir is formed by two rows of longitudinal ribs. The ribs of the internal row originate from the flame cell, external ribs are formed by the proximal canal cell. Each external rib has a remarkable bundle of microfilaments, originating in the cytoplasm of the first canal cell distally to the bases of external ribs. Membrane of internal ribs is marked by small electrondense granules, separate or fused to an electron-dense layer, continuous to dense “membrane,” connecting both external and internal ribs. Sparse internal leptotrichs originate from the bottom of the flame bulb cavity. External leptotrichs are lacking. Septate junction is present only in proximal canal cell at the level of tips of cilia. The apical surface of the canal cell bears rare short microvilli. The basal membrane of canal cells forms long invaginations that may reach nearly the apical membrane. The epithelium of excretory canals lacks the cilia. The ultrastructure of flame bulbs and epithelium of the excretory canals in B. semperi shares representatives of suborder Proseriata (Seriata). The contradiction exists in interpretation of the structure of flame bulbs in Proseriata. Ehlers and Sopott-Ehlers assumed that the external ribs are derivatives of the proximal canal cell and internal ones are outgrowths of the terminal cell, while Rohde has found conversely: the external ribs are outgrowths of the terminal cell, the internal ones are outgrowths of the proximal canal cell. However, the illustrations provided by Rohde do not enable to ascertain what cells the internal and external ribs derive from, while illustrations provided by Ehlers justify his interpretation. The order of weir formation in B. semperi confirms the viewpoint of Ehlers. The implication of ultrastructure of flame bulbs in Proseriata, especially of the order of flame bulb formation, in the Platyhelminthes phylogeny has been discussed.     

Venom and Dufour's glands of the emerald cockroach wasp Ampulex compressa (Insecta,Hymenoptera, Sphecidae): Structural and biochemical aspects

The digger wasp species Ampulex compressa produces its venom in two branched gland tubules. They terminate in a short common duct, which is bifurcated at its proximal end. One leg is linked with the venom reservoir, the other one extends to the ductus venatus. Each venom gland tubule possesses, over its entire length, a cuticle-lined central duct. Around this duct densely packed class 3 gland units each composed of a secretory cell and a canal cell are arranged. The position of their nuclei was demonstrated by DAPI staining. The brush border of the secretory cells surrounds the coiled end-apparatus. Venom is stored in a bladder like reservoir, which is surrounded by a thin reticulated layer of muscle fibres. The reservoir as a whole is lined with class 3 gland units. The tubiform Dufour's gland has a length of about 350 μm (∅ 125 μm) only and is surrounded by a network of pronounced striated muscle fibres. The glandular epithelium is mono-layered belonging to the class 1 type of insect epidermal glands. The gland cells are characterized by conspicuous lipid vesicles. Secretion of material via the gland cuticle into the gland lumen is apparent. Analysis of the polypeptide composition demonstrated that the free gland tubules and the venom reservoir contain numerous proteins ranging from 3.4 to 200 kDa. The polypeptide composition of the Dufour's gland is completely different and contains no lectin-binding glycoproteins, whereas a dominant component of the venom droplets is a glycoprotein of about 80 kDa. Comparison of the venom reservoir contents with the polypeptide pattern of venom droplets revealed that all of the major proteinaceous constituents are secreted. The secreted venom contains exclusively proteins present in the soluble contents of the venom gland. The most abundant compound class in the Dufour's gland consisted of n-alkanes followed by monomethyl-branched alkanes and alkadienes. Heptacosane was the most abundant n-alkane. Furthermore, a single volatile compound, 2-methylpentan-3-one, was identified in various concentrations in the lipid extract of the Dufour's gland.     

The ultrastructure of the female accessory gland,the cement gland,in the insect Rhodnius prolixus

The cement gland of Rhodnius prolixus is an epidermally derived tubular gland consisting of a distal synthetic region and a proximal muscular duct region. The synthetic region consists of numerous secretory units joined to a central chitinous duct via cuticular ductules. Proteinaceous secretion, synthesized by the goblet-shaped secretory cell, passes through the delicate cuticular lattice of a ductule-end apparatus and out through fine ductules to the central duct. Secretory cells are rich in rough endoplasmic reticulum and mitochondria. Light microscopy, SEM and TEM reveal the delicate lattice-like end apparatus structure, its formation and relationship to the secretory cell. The secretory cell associates via septate junctions with a tubular ductule cell that encloses a cuticle-lined ductule by forming an elaborate septate junction with itself. The ductules are continuous with the cuticle lining of the large central duct that conveys secretion to the proximal area. The proximal muscular duct has a corrugated cuticular lining, a thin epithelium rich in microtubules and thick longitudinal, striated muscles which contract during oviposition, forcing the secretion out. Histochemistry and electrophoresis reveal the secretion as proteinaceous.     

Ultrastructure of polymorphicMucor as observed by means of freeze-etching

Dormant sporangiospore ofMucor was observed by means of freezeetching. There was considerable inter- and intraspecies variation in spore size. Large spores were clearly multinucleate. The spore wall was covered with two thin layers, each about 10 nm thick, which may correspond to ordinary spore sheath. However, fracture never occurred along the spore surface. The cell membrane did not have invaginations like those of higher fungi. Instead, there were numerous round depressions about 50 nm in diameter. They revealed a small hollow when crossfractured. Occasionally they combined to form a structure resembling a rod-like invagination. This, presumably, is one step towards generation of the rod-like invagination of conidiospore. Mitochondria became much larger than those found in the vegetative forms, and showed wide and deep invaginations of membrane. Cristae became indistinct. Lipid droplets had multilayered shells and were much more highly developed than those found in mycelial cells, yeast or arthrospores of this organism. No endoplasmic reticulum or vacuoles were found.     

Ultrastructure of the spines and neck gland of Abananote hylonome Doubleday, 1844 (Lepidoptera: Nymphalidae)

The external morphology of the cuticular spines, and the ultrastructure of the spines and neck gland in fifth instar Abananote hylonome larvae was studied. The larvae are spiny along the length of their bodies. Along the length of the spines are setae with a swelling towards the apical region. Internally, in the base of each seta there is a complex of secretory cells surrounding a large vacuole continuous with the seta. The neck gland is eversible, composed of a pair of oval internal sacks connected to the exterior via an extracellular canal produced by an invagination of the cuticle. The sack cells surround a reservoir containing an amorphous substance. In both the spines and neck gland the nuclei are large and irregularly shaped, typical of defensive glands of Lepidoptera. The border of the cells adjacent to the vacuoles (spines) and the reservoir (neck gland) is made up of numerous microvilli. We suggest that defensive compounds are produced in the gland cells and then later released via the vacuoles in the spines and the extracellular canal in the neck gland.     

Comparing the secretory pathway in honeybee venom and hypopharyngeal glands

We provide insights into the secretory pathway of arthropod gland systems by comparing the royal jelly-producing hypopharyngeal glands and the venom-producing glands of the honeybee, Apis mellifera. These glands have different functions and different product release characteristics, but both belong to the class 3 types of insect glands, each being composed of two cells, a secretory cell and a microduct-forming cell. The hypopharyngeal secretory cells possess an extremely elongate tubular invagination that is filled with a cuticular structure, the end-apparatus, anchored against the cell membrane by a conspicuous series of actin rings. In contrast, venom glands have no actin rings, but instead have an actin-rich brush border surrounding the comparatively short and narrow end-apparatus. We relate these cytoskeletal differences to the production system and utilisation of secretions; venom is stored in a reservoir whereas royal jelly and enzymes are produced on demand. Fluorescence-based characterisation of the actin cytoskeleton combined with scanning electron microscopy of the end-apparatus allows for detailed characterisation of the point of secretion release in insect class 3 glands.     

Ultrastructure,functional morphology and evolution of recto-canal epidermal glands in Myriapoda

In Chilopoda, solitary epidermal glands are composed of a couple of cells only. These glands are highly abundant on the entire body surface and are distributed throughout the single-layered epidermis. Some authors provided more or less comprehensive observations on the structure of epidermal glands of specific chilopod taxa. However, no information is hitherto available on the ultrastructural diversity of these glands. Furthermore, potential homologies of these chilopod epidermal glands and of their characteristic cellular components remain unknown. Based on our results, we are now able to distinguish two types of epidermal glands in Chilopoda that can be clearly distinguished by their structure and the course of their conducting canal: recto-canal epidermal glands (rceg) and flexo-canal epidermal glands (fceg). In the present paper, we focus on the rceg. We examined the ultrastructural organization of these glands in the head region and on the anterior trunk segments of various representatives of the five extant chilopod orders by light- and electron-microscopy. According to our terminology, rceg consist of up to five different cell types including: a) distal canal cells, b) proximal canal cells, c) intermediary cells, and d) two different types of secretory cells. Intermediary and canal cells form a common conducting canal. The rceg may taxon-specifically differ in relative size and subcellular architecture, but all have the following features in common: 1) a wide distribution on various body regions among all five chilopod subtaxa, 2) the straight, broad and locally dilated conducting canal surrounded by closely packed microvilli or microvilliform infoldings around the apex of the canal cell(s), and 3) the tendency to aggregate to form compound glandular organs of massive size and complexity. Tricellular glandular units established by three different cell types are observed in Scutigeromorpha and Geophilomorpha, whereas four cell types constitute rceg in Lithobiomorpha and Craterostigmomorpha. Five different cell types per glandular unit are found only in Scolopendromorpha. The partial cuticularization of the lower part of the conducting canal formed by the intermediary cell, as found in Chilopoda, differs from the pattern described for equivalent euarthropod epidermal glands, as for instance in Hexapoda. Their wide distribution in Chilopoda and Progoneata makes it likely that tricellular rceg were at least present in the last common ancestor of the Myriapoda. Concerning Chilopoda, the evolution of highly diverse rceg is well explained on the basis of the Pleurostigmophora concept. Glands of the recto-canal type are also found in other arthropods. The paper discusses cases where homology of rceg and also fceg may be assumed beyond Myriapoda and briefly evaluates the potentials and the still-to-be-solved issues prior to use them as an additional character system to reconstruct the phylogeny of the Euarthropoda.     

Morphology of the pronotal compound glands in <Emphasis Type="Italic">Tritoma bipustulata</Emphasis> (Coleoptera: Erotylidae)

Members of the cucujiform family Erotylidae possess a whole arsenal of compound integumentary glands. Structural details of the glands of the pronotum of Tritoma bipustulata and Triplax scutellaris are provided for the first time. These glands, which open in the posterior and anterior pronotal corners, bear, upon a long, usually unbranched excretory duct, numerous identical gland units, each comprising a central cuticular canal surrounded by a proximal canal cell and a distal secretory cell. The canal cell forms a lateral appendix filled with a filamentous mass probably consisting of cuticle, and the cuticle inside the secretory cell is strongly spongiose—both structural features previously not known for compound glands of beetles. Additional data are provided for compound glands of the prosternal process and for simple (dermal) glands of the pronotum. A combined defense plus anti-microbial function of the compound glands is tentatively proposed.