Trachea Diffusion of oxygen and carbon dioxide takes place across the walls of the smallest tubes, called tracheoles, which penetrate tissues and even indent individual cells. Gas may be conducted through the respiratory system by means of active ventilation or passive diffusion. Unlike vertebrates, insects do not generally carry oxygen in their haemolymph. This is one of the factors that may limit their size. A tracheal tube may contain ridge-like circumferential rings of taenidia in various geometries such as loops or helices. Taenidia provide strength and flexibility to the trachea. In the head, thorax, or abdomen, tracheae may also be connected
What Work Is terribly damaged conditions of American labor, whose circumstance has perhaps never been more wrecked.” After Levine was named Poet Laureate, "What Work Is" reached the top of Amazon.com's "Movers and Shakers" list. What Work Is What Work Is is a collection of poetry by Philip Levine. The collection has many themes that are representative of Levine's writing including physical labor, class identity, family relationships and personal loss. Its primary focus on work and the working class led to it being studied with emphasis on Marxist literary criticism. The focus on work is expressed in thematically different ways throughout the collection.