In order to demonstrate the type of images obtainable using magnetic resonance microscopy on typical entomological specimens we imaged a dead queen of the common wasp (Vespula vulgaris, collected prior to nest founding in spring, 2002) and a live worker of the large ant species Dinoponera quadriceps. To limit movement during imaging, the D. quadriceps specimen was refrigerated at 5 °C for 15 minutes and then wrapped tightly in tissue paper. The specimen was then put into a 15mm (internal diameter) glass NMR sample tube that was loaded into the bore of the magnet, which was chilled to between 10.5 µC and 13 µC. Imaging data were collected at the Magnetic Resonance Centre (School of Physics and Astronomy) of the University of Nottingham on a Bruker DSX 400MHz spectrometer (http://www.bruker.com/) equipped with a super-wide bore 9.4 Tesla magnet and standard Bruker microimaging accessories. Pulse sequences were either a conventional 2D spin-echo image sequence or a 3D spin-echo sequence with short echo time. Parameter settings for individual images are indicated on the appropriate legend. The data were transformed and slices produced using a Silicon Graphics workstation and image processing software (IDL, RSI Systems, http://www.rsinc.com/). Three-dimensional images were produced with volume rendering as implemented in the Voltex module of Amira 2.3, a 3D visualization and reconstruction software package running on Windows 9x/ME or Windows NT/2000.
Images were obtained in frontal and transverse sections. Transverse sections were used to prepare three-dimensional reconstructions. Three-dimensional reconstructions and successive-slice movies are available at the hyperlink above.
The transverse sections (Fig 1A) through the head, thorax and abdomen show many internal structures clearly. In the head (Fig. 1B), the ocelli, protocerebral lobe, optic lobe, optic nerve, eye, mandibular muscles and oesophagus are all visible. The musculature of the thorax is particularly well resolved with the longitudinal and vertical indirect wing muscles prominent (Fig. 1C). Thoracic air spaces of the tracheal system are also visible. In the mid abdomen (Fig. 1D), the ventriculus and hindgut of the digestive system and the ovaries are both apparent. It is possible to discern fine details of the ventricular musculature and individual oocytes within the ovaries. As in the thorax, tracheal air sacs can be seen. In the lower abdomen (Fig. 1E), the poison gland can be seen as well as the ventriculus and hindgut.
The three-dimensional reconstructions (Figs. 3A-E) allow organs to be seen in situ and show spatial relationships and surface texture not apparent on the transverse slices. The gut (Figs. 3C and 3D) and thoracic musculature (Fig. 3A) are particularly striking in these reconstructions. It is possible to color parts of each section manually to provide a three-dimensional colored image of particular organs and organ systems (Fig. 3B). Structures may also be removed in this way to improve clarity and the whole image, or a part of it, may be rotated onscreen. Surface reconstructions and volume calculation can be performed by manually outlining individual organs.
Only the clearest section (a sagittal section) obtained from a live D. quadriceps is shown (Fig. 2). Internal structures are poorly resolved although the gut is partly discernable. This shows that imaging of live insects can still be problematic, unless one has an efficient method for preventing internal organ movements.