Myriapods are members of the subphylum Myriapoda, which includes arthropods like millipedes and centipedes. Myriapods typically have a single pair of antennae and simple eyes. The exceptions are the two classes symphylans and pauropods, as well as the millipede orders Polydesmida and Geophilomorpha, which are all eyeless. The compound eyes of house centipedes, on the other hand, are large and well-developed. Malpighian tubules expel nitrogenous waste into the digestive system, which is typically a simple tube. Although each segment of the ventral nerve cord has a ganglion, the brain is still in its infancy. Myriapods are most common in moist forests, where they play an important role in decomposing decaying plant material, though a few can be found in grasslands, semi-arid habitats, and even deserts. With the exception of centipedes, which are primarily nocturnal predators, the majority are detritivorous. A few centipede and millipede species can produce light and are thus bioluminescent. Pauropodans and symphylans are small, sometimes microscopic animals that live in soil and resemble centipedes on the surface. Millipedes are distinguished from the other groups by the fact that their body segments are fused into pairs, giving the appearance that each segment has two pairs of legs, whereas the other three groups have a single pair of legs on each body segment.