THE STRUCTURE AND FORMATION OF THE EGG OF THE FRUIT TREE RED SPIDER MITE, <i>METATETRANYCHUS ULMI</i> KOCH

Abstract

The summer egg, and the overwintering diapausing egg of the fruit tree red spider mite, Metatetranychus ulmi Koch are described in detail; there is no reliable factor by which these two may be distinguished on inspection. Both types of egg have a common basic shell structure, consisting of an outer thick wax layer of very high melting point and a cement layer of oil and protein which attaches the wax to the underlying ‘shell’ layer enclosing the living material. The shell layer is extremely resistant to penetration and to attack by chemicals or solvents; it appears to be composed of a material similar to keratin. This layer is formed in the ovary, towards the end of yolk accumulation. The egg is apparently fertilized precociously, and there are no associated nurse or follicle cells; only one egg is matured at a time. The female reproductive system is described in some detail; the egg receives its shell layer in the simple sac‐like ovary, and then passes into a glandular ovipositing pouch which is evaginated through the genital aperture at oviposition. The shell layer makes contact with the substrate, and the pouch secretes the cement over the rest of the shell layer so that the egg adheres to the substrate by a ring of cement around the base. The outer wax is then secreted over the cement, and it also leaves large hole at the base of the egg, bordered by the cement. The female leaves the egg by rotating into an almost vertical position. In this process the ovipositing pouch is drawn off the egg, and withdrawn into the female; consequently, the wax, which is plastic when first secreted, is drawn up into the characteristic spike which surmounts the egg. The spike is of no further physiological significance to the egg. Owing to the hole in the egg‐coverings at the base, the shell structure is not waterproof. The developing organism waterproofs the summer egg by secreting wax layer into the inside of the shell about 6 hr. before laying, previous to which the egg will only survive in humidities greater than 85 yo R.H. After waterproofing, the egg readily develops in humidities of 30 yo R.H. The waterproofing wax has a transition point of 68°C. The shell of the winter egg is similarly composed and waterproofed, but winter eggs are held up in the female until a later stage of embryonic development, so that they are waterproofed when they are deposited on the bark. Some discussion is made of the importance of leaf and bark humidities in relation to the different times at which summer and winter eggs are waterproofed.