Cactus species are well represented in arid and semiarid environments, where they sometimes even dominate the landscape, forming true cactus forests. In this review, we summarize for the first time the importance of cactus species as a wood resource for a neglected saproxylic (dead wood-dependent) entomofauna associated with decomposing and decomposed tissues in arid and semiarid environments. The complex decomposition process of cactus species is discussed in depth. We provide a checklist of the entomofauna living in decaying and decayed cacti, in which insects represented 93% of the species recorded. In particular, Coleoptera (41.5%) and Diptera (43.8%) orders together represented 85% of the insect species recorded. The family Drosophilidae made up 50% of studied Diptera species because the drosophilid–cactus–yeast complex has been widely studied during recent decades. However, we found that during the last 2 decades, there has been increasing interest in investigating the diversity of arthropods in this saproxylic habitat in Mexico, with a special focus on Syrphidae (Diptera) and Coleoptera, the former for their role in the decomposition process. Disentangling the role of the insects associated with decaying cactus is a challenge when it comes to understanding the function of xeric saprophagous insects in this environment. One frontier in this research area is to find whether decomposed stems in xeric soils would act as “islands” of fertility due to the amount of nitrogen which could be recuperated from these microecosystems. Further research is needed to understand the ecological succession of the entomofauna playing a role in the decomposition process of cactus species in semiarid environments.