The physiological function of an organ depends upon its shape and position within the body. Laetitia Bataillé and colleagues now provide insight into how the organs of the Drosophila larvae are positioned to function properly, thanks to two poorly understood muscle types – the alary muscles (AMs), known to connect the exoskeleton to the dorsal vessel (the fly heart), and the recently discovered thoracic alary-related muscles (TARMs), which link the exoskeleton to the midgut. The authors first reveal that the attachment of AMs and TARMs to either circulatory or visceral systems is under the control of Hox genes. Fluorescent transgenes in intact animals then reveal the connections that AMs and TARMs make between the epidermis and multiple internal organs. The muscles are multinucleated and sarcomeric, and are simultaneously highly deformable and contractile. Genetic ablation of TARMs and AMs induces strong lethality. Ablation of AMs causes collapse of the dorsal vessel and heart lumen – demonstrating a requirement to suspend the heart inside the body cavity – and also leads to mis-positioning of the tracheal trunk. Ablation of TARMs changes the position of the gut and gastric caeca, which reduces the amount of food that transits through the midgut. AMs and TARMs are thus a new type of muscle connecting the insect exoskeleton to multiple internal organs and ensuring the proper positioning and physiological function of these organs.
