A. A. Panov

Structure of the mushroom bodies in Scarabaeoidea (Coleoptera): 2. Phytophagous Scarabaeidae and general discussion

The mushroom body structure is progressively more complicated within the superfamily Scarabaeoidea. In the basal families and coprophagous Scarabaeidae, the calyx region is asymmetrical and poorly developed but reaches the maximal degree of development in the phytophagous Scarabaeidae of the subfamilies Melolonthinae and Cetoniinae. These Scarabaeidae have two separate calyces; moreover, some species have additional regions of glomerular neuropile. The processes of different Kenyon cell types segregate into special subdivisions of vertical and medial lobes. It is assumed that a progressive development of the mushroom body in phytophagous Scarabaeidae is associated with the need to integrate olfactory and visual information.

[Leaf beetles (Coleoptera: Chrysomelidae): mushroom body simplification in the course of progressive evolution of the family].

Members of different subfamilies of Chrysomelidae differ strongly in the degree of mushroom body development. The mushroom bodies are especially strongly developed (with the calyx in the form of large cups and double shafts of the peduncular apparatus) in the evolutionarily primitive subfamilies Sagrinae and Criocerinae, and considerably reduced in members of more evolved subfamilies, with the calyx region weakly developed and shafts of the peduncular apparatus fused together. It is suggested that this mushroom body reduction can be related to the closer connection of the head with the prothorax, which is found in the more evolved leaf beetle subfamilies.

Longicorn beetles (Coleoptera: Cerambycidae) differ considerably in the degree of their mushroom body development

A duality in the general structure of the mushroom body in longicorn beetles is confirmed. This duality is associated with the fact that they are formed by two solitary neuroblasts or two neuroblast clusters on each side of the brain and are manifested as a bipartite structure of both the calyx, which is the main sensory input, and the peduncular apparatus. Within the studied longicorn beetles, modifications in the general structure of mushroom bodies have been found; these modifications are caused by two oppositely directed morphogenetic processes, namely, the concentration of structures and their compartmentalization. The concentration leads to disappearance of the bipartite structure of the peduncular apparatus, whereas compartmentalization leads to a secondary subdivision of these structures into anatomically distinct subsections. This process is most pronounced in the peduncle and lobes. The mushroom bodies are best developed and differentiated in the members of the subfamily Lamiinae.

General structure of the mushroom body calyx in brachycera orthorrhapha flies (Diptera)

The mushroom body calyx in Brachycera Orthorrhapha flies is extremely diverse in the degree of development. In general, the calyx has the anterior, posterior, and dorsal lobes, as well as “sleeves” of glomerular neuropil surrounding Kenyon cell fibers. The anterior lobe of the calyx is found in all species studied. The most complex structure of the calyx is characteristic of the families Empididae and especially Bombyliidae, in which it has all the parts listed above. Brachycera Orthorrhapha flies have three fiber bundles of Kenyon cells, in contrast to four bundles in Drosophila. It is assumed that each mushroom body in Brachycera Orthorrhapha flies is formed by descendants of three single neuroblasts.

Structure of the mushroom bodies in Scarabaeoidea (Insecta: Coleoptera): 1. Basal families and coprophagous scarabaeidae

Mushroom bodies are in general similarly developed in most taxons studied. The calyx region appears as a single structure, and its dual nature is not yet realized. An anterio-posterior asymmetry of the calyx region with Kenyon cell processes running mostly behind the glomerular neuropil of the calyx is characteristic of all the species studied. In this respect, the calyx region of basal Scarabaeoidea resembles greatly the calyx of many dipterans. Lobe compartmentalization occurs at the initial stage. The passalid beetles represent an exception, as their mushroom bodies are much more developed than in related families. This may be connected with the complicated social behavior of Passalidae.

How many neuroblasts build mushroom bodies in Lucilia caesar L. and Musca domestica L. (Diptera, Brachycera Cyclorrhapha)?

The green-bottle fly Lucilia caesar and the housefly Musca domestica differ greatly in the number of neuroblasts producing mushroom bodies. Four neuroblasts were found in each mushroom body of Lucilia pupae, and its calyx has a quadruple structure. In the housefly, the number of mushroom body neuroblasts rises up 20 in each brain hemisphere. This leads to a more complicated calyx structure. The neuroblast number observed in Lucilia and Musca is compared with that found in other Diptera.

[Histological structure of tripartite mushroom bodies in ground beetles (Insecta, Coleoptera: Carabidae)].

Contrary to members of the suborder Polyphaga, ground beetles have been found to possess tripartite mushroom bodies, which are poorly developed in members of basal taxa and maximally elaborated in evolutionarily advanced groups. Nevertheless, they do not reach the developmental stage, which has been previously found in particular families of beetles. It has been pointed out that a new formation of the Kenyon cells occurs during at least the first months of adult life, and inactive neuroblasts are found even in one-year-old beetles. It has been suggested that there is a relation between the Kenyon cell number and development of the centers of Kenyon cell new-formation.

Diversity in neuroblast number forming mushroom bodies of the higher dipterans (Insecta, Diptera, Brachycera Cyclorrhapha)

Neurogenesis in mushroom bodies is studied in 12 species of the higher dipterans. A significant difference in the number of neuroblasts forming mushroom bodies is found. In the majority of species studied, Kenyon cells are formed by four solitary neuroblasts. Among six calliphorid species, the number of neuroblasts increases up to 10–15 (mean 12.6) in each mushroom body in Calliphora vicina only. In young pupae of Muscina stabulans and M. livida, four polyneuroblastic prolipherative centers occur instead of solitary neuroblasts. These centers disintegrate later into numerous solitary neuroblasts. A hypothesis on the origin of the polyneuroblastic structure of mushroom bodies found in C. vicina and, earlier, in Musca domestica, is proposed.

General brain structure of newly hatched larva and neuroblasts in larval mushroom bodies in Pterostichus niger Deg. (Coleoptera: Carabidae)

It is revealed that the larval brain of Pterostichus niger, an active predator with well-developed long-distance sense organs (the set of antennal sensilla and lateral ocelli, or stemmata) at hatching already contains optic lobes, which include two groups of optic neuropils and complex antennal lobes of glomerular neuropil. It is shown that the central complex of the protocerebrum is represented by a bipartite protocerebral bridge and the upper part of the central body and the mushroom bodies include numerous Kenyon cells, a well-developed calyx, a peduncular apparatus, and numerous neuroblasts.

A novel, unusual (at least for beetles) mode of Kenyon cell production in the diving beetle Cybister lateralimarginalis Deg. (Coleoptera: Dytiscidae)

Kenyon cell production in the mushroom bodies of Cybister lateralimarginalis is a peculiar process. It has been found that each proliferative center contains one giant neuroblast, which divides unequally, and its smaller daughter cell becomes the 2nd order neuroblast dividing unequally as well. The smaller daughter cell of this neuroblast becomes a ganglion mother cell. The latter, as usual, divides equally producing two Kenyon cells.