Box 3.2. Tracheal hypertrophy in mealworms at low oxygen concentrations
Resistance to diffusion of gases in insect tracheal systems arises from the spiracular valves when they are partially or fully closed, the tracheae, and the cytoplasm supplied by the tracheoles at the end of the tracheae. Air-filled tracheae will have a much lower resistance per unit length than the watery cytoplasm because oxygen diffuses several orders of magnitude faster in air than in cytoplasm for the same gradient of oxygen partial pressure. Until recently, the tracheal system was believed to provide more than sufficient oxygen (at least in non-flying insects that lack air sacs), with the tracheae offering trivial resistance to the passage of oxygen.
Experiments on mealworm larvae, Tenebrio molitor (Coleoptera: Tenebrionidae), that were reared in different levels of oxygen (all at the same total gas pressure) showed that the main tracheae that supply oxygen to the tissues in the larvae hypertrophy (increase in size) at lower oxygen levels. The dorsal (D), ventral (V), and visceral (or gut, G) tracheae were affected but not the lateral longitudinal tracheae that interconnect the spiracles (the four tracheal categories are illustrated in an inset on the graph). The dorsal tracheae supply the dorsal vessel and dorsal musculature, the ventral tracheae supply the nerve cord and ventral musculature, whereas the visceral tracheae supply the gut, fat body, and gonads. The graph shows that the cross-sectional areas of the dorsal, ventral, and visceral tracheae were greater when the larvae were reared in 10.5% oxygen (black dot) than when they were reared in 21% oxygen (as in normal air) (white dot) (after Loudon 1989). Each point on the graph is for a single larva and is the average of the summed areas of the dorsal, ventral, and visceral tracheae for six pairs of abdominal spiracles.
This hypertrophy appears to be inconsistent with the widely accepted hypothesis that tracheae contribute an insignificant resistance to net oxygen movement in insect tracheal systems. Alternatively, hypertrophy may simply increase the amount of air (and thus oxygen) that can be stored in the tracheal system, rather than reduce resistance to air flow. This might be particularly important for mealworms because they normally live in a dry environment and may minimize the opening of their spiracles. Whatever the explanation, the observations suggest that some adjustment can be made to the size of the tracheae in mealworms (and perhaps other insects) to match the requirements of the respiring tissues