Chapter 5 – RESPIRATION—AERIAL GAS TRANSPORT

4 – Structure of the Digestive System

Is the remarkable diversity in the behavior of insects reflected in the organization of their nervous systems? The ventral nerve cords (VNCs) have been described from over 300 insect species covering all the major orders. Interpreting these data in the context of phylogenetic relationships reveals remarkable diversity. The presumed ancestral VNC structure is rarely observed; instead the VNCs of most insects show extensive modification and substantial convergence. Modifications include shifts in neuromere positions, their fusion to form composite ganglia, and, potentially, their separation to revert to individual ganglia. These changes appear to be facilitated by the developmental and functional modularity of the VNC, a neuromere for each body segment. The differences in VNC structure emphasize trade-offs between behavioral requirements and the costs incurred while maintaining the nervous system and signaling between its various parts. The diversity in structure also shows that nervous systems may undergo dramatic morphological changes during evolution.

Diversity and evolution of the insect ventral nerve cord.

This work presents a critical evaluation of the currently available morphological and biological data base relevant to reconstructing phylogeny in Dictyoptera. In 1992, B.L. Thorne and J.M. Carpenter (Systematic Entomology 17: 253-268) pub- lished a widely acknowledged phylogenetic analysis of Dictyoptera that gave the relationships (Mantodea + (*Blattaria + Cryptocercidae)) + ((Mastotermitidae + Kalotermitidae) + Termopsidae) (*Blattaria = Blattaria excluding Cryptocercidae). The present study uses that work as a starting point to demonstrate the importance of a thorough treatment of characters in phyloge- netic reconstruction. Repeating the analysis of Thorne and Carpenter without disputable polarity assumptions produced Mantodea + ((*Blattaria + Cryptocercidae) + ((Mastotermitidae + Kalotermitidae) + Termopsidae)). Analyses after a careful revision of the characters as well as analyses including seven additional characters produced Mantodea + (*Blattaria + (Cryptocercidae + (Mastotermitidae + (Kalotermitidae + Termopsidae)))). The latter result, indicating paraphyly of Blattaria with respect to Isoptera, is in strong contrast to the findings of Thorne and Carpenter, but concurs with some recent morphological and molecular studies. It includes a single acquisition of both symbiotic fat body Blattabacterium and hindgut flagellates within Dictyoptera, and the homology of sociality in Cryptocercidae and Isoptera.

Phylogeny of the Dictyoptera Re-examined (Insecta)

Antibiotic feeding studies were conducted on the firebrat,Thermobia domestica (Zygentoma, Lepismatidae) to determine if the insect's gut cellulases were of insect or microbial origin. Firebrats were fed diets containing either nystatin, metronidazole, streptomycin, tetracycline, or an antibiotic cocktail consisting of all four antibiotics, and then their gut microbial populations and gut cellulase levels were monitored and compared with the gut microbial populations and gut cellulase levels in firebrats feeding on antibiotic-free diets. Each antibiotic significantly reduced the firebrat's gut micro-flora. Nystatin reduced the firebrat's viable gut fungi by 89%. Tetracycline and the antibiotic cocktail reduced the firebrat's viable gut bacteria by 81% and 67%, respectively, and metronidazole, streptomycin, tetracycline, and the antibiotic cocktail reduced the firebrat's total gut flora by 35%, 32%, 55%, and 64%, respectively. Although antibiotics significantly reduced the firebrat's viable and total gut flora, gut cellulase levels in firebrats fed antibiotics were not significantly different from those in firebrats on an antibiotic-free diet. Furthermore, microbial populations in the firebrat's gut decreased significantly over time, even in firebrats feeding on the antibiotic-free diet, without corresponding decreases in gut cellulase levels. Based on this evidence, we conclude that the gut cellulases of firebrats are of insect origin. This conclusion implies that symbiont-independent cellulose digestion is a primitive trait in insects and that symbiont-mediated cellulose digestion is a derived condition.

/pdf/cellulose-digestion-in-primitive-hexapods-effect-of-ingested-568p50ppo9.pdf

Cellulose digestion in primitive hexapods: Effect of ingested antibiotics on gut microbial populations and gut cellulase levels in the firebrat, Thermobia domestica (Zygentoma, Lepismatidae)

The several organ systems of these two species are described, compared, and illustrated. Differences were found in the tracheation of the eye, the disposition of vestigial stylal tracheae, and the absence in L. saccharinum of the ventral tracheal commissure of the eighth abdominal segment. The digestive system follows a generalized plan, similar to that of Orthoptera. In C. campbelli , but not in L. saccharinum , the first five abdominal ganglia have migrated one segment forward, but each continues to innervate the segment from which it came, whether a tracheal connection with that segment is preserved or not; the different behavior of tracheae and nerves in relation to such migrated ganglia seems a result of the sequence of their formation in the embryo. In both species the dorsal vessel is pulsatile for its whole length, but little or no peristalsis was observed; instead, systoles occur alternately in an anterior and a posterior chamber which are separated by a thoracic valve. Ten pairs of ostia were observed with difficulty through the transparent integument of living C. campbelli; probably those of L. saccharinum are similar. Pericardial cells were less numerous in the former than in the latter species. Each ovary consists of five ovarioles, connected to a common duct near the base of the ovipositor; the spermatheca lies beneath the eighth abdominal ganglion and communicates with the oviduct, ovipositor, and genital pore above the eighth sternite. Males have three pairs of testes on each side of the body, each pair discharging through a common vas efferens into a vas deferens which leads to one of the two seminal vesicles; and from this a long tube, with accessory glands apparently built into it, runs to the base of the acdeagus. Apical cells, not previously known in Thysanura, were found present in the testes. Histological sections of the aedeagus and its base provide, for the first time, evidence for a paired origin of this organ in Apterygota.

Clyde Stirling Barnhart

Papers

The Internal Anatomy of the Silverfish Ctenolepisma campbelli and Lepisma saccharinum (Thysanura: Lepismatidae)