*3.1.4 Others*

In addition to lipids, neuropeptides, and proteins, MSI can also be used to visualize the distributions of defensive compounds, special proteins (e.g., venom allergens and toxins) and other small molecules (e.g., betaine and amino acids). Defensive compounds (e.g., pederin, pseudopederin, and pederon) were detected and localized in the organs of *Paederus riparius* [35]. Three venom allergens and two venom toxins were mapped in the honeybee [54]. Poison sac was the lactation of main venom proteins in *Solenopsis invicta* [59]. Nonpolar compounds (e.g., (*E*)-1-nitropentadec-1 ene and (*E*)-hex-2-enal) can be detected from the head to the abdomen in two model insects, *Prorhinotermes simplex* and *Graphosoma lineatum*. Gland openings and gland reservoirs were the most active areas in *P. simplex* and *G. lineatum* [57]. Other small molecules (e.g., betaine and amino acids) were detected in *Schistocerca gregaria* [37]. Semiochemicals were mapped on the surface of the adults of *D. melanogaster* [43]. Two male-specific sex pheromones were localized in the ejaculatory bulb of *D. melanogaster* [45]. MSI can also be used as a novel *in situ* metabonomic tool to study the metabolism of L-arginine of the honeybee brain in response to proboscis extension [53].
