Fine Structure Of The Midgut And Malpighian Tubules Of Panorpidae And Bittacidae (Mecoptera)

The midgut is in charge of the processes of digestion, absorption, secretion, storage oftoxic substances and excretion in taxa lacking Malpighian tubules in insects. Malpighiantubules are the principal organs of the excretory system lying on the junction of midgut andhindgut. Numerous researches on the fine structure of midgut and Malpighian tubules havebeen reported in insects. To our knowledge, however, the fine structure of the midgut andMalpighian tubules has rarely been investigated in Mecoptera.Mecoptera are one of the most primitive orders in holometabola insects and occupy ansignificant position in the phylogeny of insects. This group is considered as a transition groupfrom the solid feeders to liquid feeders in Insecta based on the feature of the proventriculus.In this thesis, the anatomy, morphology, histology and ultrastructure of the midgut ofPanorpa obtusa Cheng, Neopanorpa lui Chou&Ran, Sinopanorpa tincta (Navás),Furcatopanorpa longihypovalva (Hua&Cai) and Dicerapanorpa magna (Chou) inPanorpidae and Bittacus planus Cheng in Bittacidae were investigated in detail using lightmicroscopy and scanning and transimission electron microscopy; and the anatomy andmorphology of the Malpighian tubules of P. obtusa and B. planus were investigated usinglight microscopy and scanning electron microscopy. The main results are as follows:The midguts of Panorpidae and Bittacidae are white tubes without gastric caeca andprominent segmentation. Only the midgut of Bittacidae has two slight constrictions. Thestructure of midgut is similar between the male and the female in genera of Panorpidae andBittacidae except P. obtusa and F. longihypovalva.Both in Panorpidae and Bittacidae, the midgut is composed of an outer longitudinal, aninner circular muscle layer, a basal lamina, an epithelium and a lumen from the outside toinside. No peritrophic membrane was found in the midgut lumen, but a membrane in themidgut lumen of P. obtusa. Numerous nodules were observed on the surface of basal laminaof the midgut in all the species except male F. longihypovalva. We found that the nodulesonly consist of the regenerative cells through observation of the histological structure of themidgut in S. tincta.Longitudinal muscles are in serried ranks and form a membranous structure covering the inner structure of midgut except male F. longihypovalva. The diameter of circular muscles issimilar in Panorpidae. Whereas in Bittacidae, they have distinct difference and partialexpands.In female F. longihypovalva, the structure of the midgut is similar to that of other genera.But midgut of male F. longihypovalva differs markedly from the latter in morphology. Thelongitudinal and circular muscles of male F. longihypovalva are much sparser than those ofthe female and other species. The nucleuses of epithelial cells were observed clearly in theSEM micrographs. The surface of basal lamina is smooth, and without nodules.The midgut epithelium is composed of digestive, regenerative and secretory cells. Indigestive cells, the basal plasma membrane forms numerous short infoldings; the apicalplasma membrane produces large quantities of cylindrical and irregular-shaped microvilliprotruding into the lumen; evagination of the apico-lateral plasma membrane extends into theadjacent cells. The regenerative cells lying on the basal lamina are closed in the epithelium bythe other cells, and are only distributed in the nodules. The secretory cells of irregular shapecontain a mass of secretory granules at the basal cytoplasm.The midgut epithelium degenerates in a continuous manner, and epithelial cells mainlydie of apoptosis or necrosis.Malpighian tubules of Panorpidae resemble those of Bittacidae in morphology: both ofthem consist of six Malpighian tubules, which are divided into two equal groups lying on thedorsal and ventral sides of the terminal midgut; each Malpighian tubule is divided into threesegments, with the first colorless and pellucid, the second brown in Panorpidae and sepia inBittacidae and straight, the third segment similar to the second in color but wavy curved. TheMalpighian tubules in both Panorpidae and Bittacidae are composed of thin basal lamina,epithelium and lumen. Many spherical granules are present in the epithelial cells ofPanorpidae, but absent in the epithelial cells of Bittacidae. The lumen is empty in Panorpidae,but is filled with solid substances in Bittacidae.Based on the above results, we conclude that the midgut, as a function organ, hascomparatively stable structure among the Panorpidae and Bittacidae. Hereby we speculatethat midgut is of little significance in taxonomy in these two families. We also discovered thatMalpighian tubules of Bittacidae are of Hemiptera type, and Malpighian tubules ofPanorpidae are distinct from any Malpighian tubules described previously.