Why can't arthropods grow to be very large ?
Arthropods have an open circulatory system. This type of system is very inefficient and would limit the size to which an organism can attain. Their heart pumps blood to their body cavity, where the tissues of the body are bathed in blood. Diffusion occurs and substances are supplied to the cells, in a slow manner. This system works well with a smaller body plan. However, in a larger, more complex organism, a more efficient closed circulatory system would be needed to rapidly supply cells with their needs. Respiration is also less efficient in Arthropods. Tracheal tubes carry oxygen from opening in the exoskeleton called spiracles directly to cells which obtain it by diffusion. For aquatic arthropods, there are gills, however, the blood doesn't have hemoglobin to transport oxygen, as seen in humans, for instance. Therefore, although their respiratory and circulatory systems are efficient enough to meet their needs, it also places a limit on the size that Arthropods grow. Arthropods possess a tough exoskeleton that cannot stretch and thus, this can restrict growth. However, they shed or molt the exoskeleton several times during their lifespan. All of these factors affect the size that Arthropods attain.
Arthropods range in size from a few millimeters to some which are several meters in length. The reason they would find it difficult to grow very large in terms of height may be to due to the fact that they have an exoskeleton.
As an arthropod grows larger it has to get rid of the exoskeleton it has and grow a new one that is larger in size. This is to do with the fact that the exoskeleton cannot stretch to accommodate new growth. This is done using the process of molting. But with a larger size, the process would become difficult.
Also, the circulatory system of arthropods is not built to allow the flow of the fluid they have which acts like blood against gravity to a very high level. Their respiratory system is also not developed in a way that will allow them to grow very large.