Ultrastructure of the retrocerebral complex in the adult midge, chironomus riparius MG (Diptera : Chironomidae)

Abstract

The organisation of the retrocerebral complex of adult Chironomus riparius Mg. (Diptera : Chironomidae) has previously been compared with that of the larva. The primary objective of the present work was to examine the ultrastructure of the components of the complex in neoimaginal male C. riparius to enable their further comparison with similar structures in other dipterans and determine whether this information supports the contention that the midge has a relatively primitive neuroendocrine system. The hypocerebral ganglion has a complex innervation, is composed of non- neurosecretory neurons and glial cells and, with the adjacent aortic wall, is the major neurohaemal site of the complex. The corpora cardiaca are not intimately associated with the ganglion. They are not apparently neurohaemal but are composed of cells containing granules with a bimodal size distribution. The corpora allata are of the pseudolymphoid type and are devoid of extracellular stromal channels. Peritracheal tissue persists into the neoimaginal midge but is apparently inactive and undergoing degeneration. The post-cerebral glands have been included in all previous accounts of the retrocerebral complex in chironomids but the present work confirms the view that they are not endocrine in nature despite their close proximity to the corpora cardiaca. Their ultrastructural appearance suggests that they may be functionally related to pericardial cells in other insects. It is concluded that the retrocerebral complex is indeed rather primitive both in its organisation and in the structure of its components.