Insects can open and close these spiracles, to conserve water that would otherwise be lost to evaporation from the open tubes. Their reliance on diffusion for respiration is one of the reasons that insects are small. Arthropods excrete by means of malphigian tubules , projections of the digestive tract that help conserve water. Terrestrial forms excrete nitrogen as uric acid , as do birds. Their waste is nearly dry, a superb adaptation to life on land.
Arthropods have an open circulatory system, and separate sexes.
Fertilization is usually internal, another adaptation for terrestrial life. Males and females often show pronounced sexual dimorphism . They are often modified as fangs or pincers. Chelicerates lack antennae. Horseshoe crabs have larvae that are very similar to trilobites, and they may be descendants of this long vanished group.