322 FOREST ENTOMOLOGY. 



one generation, the 3rd or emigrans generation, belongs exclusively to 

 conifers other than the spruce (such as Scots pine, larch, and silver 

 fir). This change of conifers referred to other than spruce is known 

 as the intermediate conifer (Zwischenkonifere) only in so far as the 

 former brings about the transport of the species, through the laying of 

 eggs on the intermediate conifer, in order that they may return to the 

 spruce. Now this is a most important point, as it clearly implies that 

 this alternation is for the welfare of the species, though, as it were, on 

 the part of the insect, not altogether optional, but periodically (not 

 annually) compulsory for the welfare of the species. This shows that 

 it must have been difficult to discover and ratify by those Continental 

 workers, and even difficult for the student in this country to verify. 



The following is a short account of the alternation as given by 

 Niisslin : l 



"The 1st generation {foundress) is a parthenogenetic female, wing- 

 less, with three-jointed antennae, confined to the spruce and producing 

 the spruce gall. The 2nd generation {migrans alata) acquires wings 

 after the fourth moult, receives at the same time five-jointed antennae, 

 compound eyes, and three simple eyes. It develops in the gall on the 

 spruce, and flies usually to one of the ' intermediate conifers,' where it 

 lays its eggs on the needles. From these eggs springs the 3rd genera- 

 tion {emigrans), which resembles its ancestor of the 1st generation (the 

 foundress). It remains on the alternate host (the ' intermediate 

 conifer') for hibernation, and lays eggs there in spring. From the 

 eggs emerge either forms resembling the parent {exulans) or the 4th 

 generation {sexupara), which in every respect resembles the generation 

 before the last (its 'grandmotherly generation'), the migrans alata, 

 only it remains smaller, and is less prolific. From the beginning it is 

 independent {freilebend), and after the first moult varies somewhat. 

 It lives and develops on the needles of the ' intermediate conifers,' 

 and flies back to the spruce, where it lays eggs on needles. Out of 

 the eggs come the two different forms of the 5th generation {sexuales) 

 the leaner males and the stouter females. Both are rather small, 

 but do not diminish in size, and have four-jointed antenna?. The 

 female, after pairing, lays in midsummer a single fertilised egg on the 

 spruce, from which emerges the hibernating foundress. The fivefold 

 cycle requires, therefore, two years and two different coniferous species 

 for its completion. 



1 Leitfaden der Forstinsektenkunde. 



