Anatomy of the light organ of the glow-worm Arachnocampa flava (Diptera: Keroplatidae)

In dipteran glow-worms (genus Arachnocampa) bioluminescence is produced by cells of the Malpighian tubules. Light is used to lure prey into sticky webs secreted by the larvae. Larvae can regulate the intensity of the emitted light so neural control of the glow is likely, either acting directly on the light-emitting cells or through regulation of oxygen access to the light organ. Here we describe the innervation, musculature, and tracheal supply in relation to the three-dimensional structure of the light organ, cryptonephridial complex and hindgut using serial section light microscopy and micro computed tomography (μCT). We also use video macrophotography to observe light emission in live larvae. Tracing of trachea in serial sections showed no structures that could actively restrict air supply to the mass of trachea termed the reflector. A bilateral pair of nerves lie alongside the Malpighian tubules and give rise to branches that innervate the hindgut and musculature. A network of muscles surround the light organ. A cryptonephridial complex anterior to the light organ is surrounded by a connective tissue sheath. The fact that the cryptonephridial complex is a plesiomorphic trait in Keroplatidae, the majority of which are not bioluminescent, suggests that the light organ is evolutionarily derived from the complex. We also propose that the cryptonephridial complex has a water conservation function in Arachnocampa because the larvae are potentially water-restricted. The three-dimensional reconstruction of the light organ provides morphological context for ongoing investigations of the biochemistry of light production and the physiological regulation of light output.